feat(iOS): Add callback for webViewWebContentProcessDidTerminate (#774)

* Add callback for WKWebView's onContentProcessDidTerminate

* Add platform information to docs

* Add new WebViewTerminatedEvent type

docs/Reference.md

 - [`onHttpError`](Reference.md#onhttperror)
 - [`onMessage`](Reference.md#onmessage)
 - [`onNavigationStateChange`](Reference.md#onnavigationstatechange)
+- [`onContentProcessDidTerminate`](Reference.md#oncontentprocessdidterminate)
 - [`originWhitelist`](Reference.md#originwhitelist)
 - [`renderError`](Reference.md#rendererror)
 - [`renderLoading`](Reference.md#renderloading)
 
 ---
 
+### `onContentProcessDidTerminate`
+
+Function that is invoked when the `WebView` content process is terminated.
+
+| Type     | Required | Platform      |
+| -------- | -------- | ------------- |
+| function | No       | iOS WKWebView |
+
+Example:
+
+```jsx
+<WebView
+  source={{ uri: 'https://facebook.github.io/react-native' }}
+  onContentProcessDidTerminate={syntheticEvent => {
+    const { nativeEvent } = syntheticEvent;
+    console.warn('Content process terminated, reloading', nativeEvent);
+    this.refs.webview.reload()
+  }}
+/>
+```
+
+Function passed to onContentProcessDidTerminate is called with a SyntheticEvent wrapping a nativeEvent with these properties:
+
+```
+canGoBack
+canGoForward
+loading
+target
+title
+url
+```
+
+---
+
 ### `originWhitelist`
 
 List of origin strings to allow being navigated to. The strings allow wildcards and get matched against _just_ the origin (not the full URL). If the user taps to navigate to a new page but the new page is not in this whitelist, the URL will be handled by the OS. The default whitelisted origins are "http://*" and "https://*".

ios/RNCWebView.m

 @property (nonatomic, copy) RCTDirectEventBlock onHttpError;
 @property (nonatomic, copy) RCTDirectEventBlock onMessage;
 @property (nonatomic, copy) RCTDirectEventBlock onScroll;
+@property (nonatomic, copy) RCTDirectEventBlock onContentProcessDidTerminate;
 @property (nonatomic, copy) WKWebView *webView;
 @end
 
   decisionHandler(WKNavigationResponsePolicyAllow);
 }
 
+/**
+ * Called when the web view’s content process is terminated.
+ * @see https://developer.apple.com/documentation/webkit/wknavigationdelegate/1455639-webviewwebcontentprocessdidtermi?language=objc
+ */
+- (void)webViewWebContentProcessDidTerminate:(WKWebView *)webView
+{
+  RCTLogWarn(@"Webview Process Terminated");
+  if (_onContentProcessDidTerminate) {
+    NSMutableDictionary<NSString *, id> *event = [self baseEvent];
+    _onContentProcessDidTerminate(event);
+  }
+}
+
 /**
  * Decides whether to allow or cancel a navigation after its response is known.
  * @see https://developer.apple.com/documentation/webkit/wknavigationdelegate/1455643-webview?language=objc

ios/RNCWebViewManager.m

 RCT_EXPORT_VIEW_PROPERTY(onLoadingProgress, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onHttpError, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onShouldStartLoadWithRequest, RCTDirectEventBlock)
+RCT_EXPORT_VIEW_PROPERTY(onContentProcessDidTerminate, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(injectedJavaScript, NSString)
 RCT_EXPORT_VIEW_PROPERTY(javaScriptEnabled, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(allowsInlineMediaPlayback, BOOL)

src/WebView.ios.tsx

   WebViewMessageEvent,
   WebViewNavigationEvent,
   WebViewProgressEvent,
+  WebViewTerminatedEvent,
   IOSWebViewProps,
   DecelerationRateConstant,
   NativeWebViewIOS,
     viewManager.startLoadWithResult(!!shouldStart, lockIdentifier);
   };
 
+  onContentProcessDidTerminate = (event: WebViewTerminatedEvent) => {
+    const { onContentProcessDidTerminate } = this.props;
+    if (onContentProcessDidTerminate) {
+      onContentProcessDidTerminate(event);
+    }
+  };
+
   componentDidUpdate(prevProps: IOSWebViewProps) {
     this.showRedboxOnPropChanges(prevProps, 'allowsInlineMediaPlayback');
     this.showRedboxOnPropChanges(prevProps, 'incognito');
         onMessage={this.onMessage}
         onScroll={this.props.onScroll}
         onShouldStartLoadWithRequest={onShouldStartLoadWithRequest}
+        onContentProcessDidTerminate={this.onContentProcessDidTerminate}
         ref={this.webViewRef}
         // TODO: find a better way to type this.
         source={resolveAssetSource(this.props.source as ImageSourcePropType)}

src/WebViewTypes.ts

 
 export type WebViewErrorEvent = NativeSyntheticEvent<WebViewError>;
 
+export type WebViewTerminatedEvent = NativeSyntheticEvent<WebViewNativeEvent>;
+
 export type WebViewHttpErrorEvent = NativeSyntheticEvent<WebViewHttpError>;
 
 export type DataDetectorTypes =
   pagingEnabled?: boolean;
   scrollEnabled?: boolean;
   useSharedProcessPool?: boolean;
+  onContentProcessDidTerminate: (event: WebViewTerminatedEvent) => void;
 }
 
 export interface IOSWebViewProps extends WebViewSharedProps {
    * @platform ios
    */
   allowingReadAccessToURL?: string;
+
+  /**
+   * Function that is invoked when the WebKit WebView content process gets terminated.
+   * @platform ios
+   */
+  onContentProcessDidTerminate: (event: WebViewTerminatedEvent) => void;
 }
 
 export interface AndroidWebViewProps extends WebViewSharedProps {