feat(WKWebview): Add incognito prop to iOS WKWebview

Allows the webview to be opened with an ephemeral data storage.

docs/Reference.md

 - [`html`](Reference.md#html)
 - [`hideKeyboardAccessoryView`](Reference.md#hidekeyboardaccessoryview)
 - [`allowsBackForwardNavigationGestures`](Reference.md#allowsbackforwardnavigationgestures)
+- [`incognito`](Reference.md#incognito)
 - [`allowFileAccess`](Reference.md#allowFileAccess)
 - [`saveFormDataDisabled`](Reference.md#saveFormDataDisabled)
 - [`pagingEnabled`](Reference.md#pagingEnabled)
 
 ---
 
+### `incognito`
+
+Does not store any data within the lifetime of the WebView.
+
+| Type    | Required | Platform      |
+| ------- | -------- | ------------- |
+| boolean | No       | iOS WKWebView |
+
+---
+
 ### `allowFileAccess`
 
 If true, this will allow access to the file system via `file://` URI's. The default value is `false`.

ios/RNCWKWebView.h

 @property (nonatomic, assign) BOOL automaticallyAdjustContentInsets;
 @property (nonatomic, assign) BOOL hideKeyboardAccessoryView;
 @property (nonatomic, assign) BOOL allowsBackForwardNavigationGestures;
+@property (nonatomic, assign) BOOL incognito;
 @property (nonatomic, assign) BOOL useSharedProcessPool;
 @property (nonatomic, copy) NSString *userAgent;
 @property (nonatomic, assign) BOOL allowsLinkPreview;

ios/RNCWKWebView.m

     };
 
     WKWebViewConfiguration *wkWebViewConfig = [WKWebViewConfiguration new];
+    if (_incognito) {
+      wkWebViewConfig.websiteDataStore = [WKWebsiteDataStore nonPersistentDataStore];
+    }
     if(self.useSharedProcessPool) {
         wkWebViewConfig.processPool = [[RNCWKProcessPoolManager sharedManager] sharedProcessPool];
     }

ios/RNCWKWebViewManager.m

 RCT_EXPORT_VIEW_PROPERTY(automaticallyAdjustContentInsets, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(hideKeyboardAccessoryView, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(allowsBackForwardNavigationGestures, BOOL)
+RCT_EXPORT_VIEW_PROPERTY(incognito, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(pagingEnabled, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(userAgent, NSString)
 RCT_EXPORT_VIEW_PROPERTY(allowsLinkPreview, BOOL)

js/WebView.ios.js

         'The allowsBackForwardNavigationGestures property is not supported when useWebKit = false',
       );
     }
+
+    if (
+      !this.props.useWebKit &&
+      this.props.incognito
+    ) {
+      console.warn(
+        'The incognito property is not supported when useWebKit = false',
+      );
+    }
   }
 
   render() {
         }
         hideKeyboardAccessoryView={this.props.hideKeyboardAccessoryView}
         allowsBackForwardNavigationGestures={this.props.allowsBackForwardNavigationGestures}
+        incognito={this.props.incognito}
         userAgent={this.props.userAgent}
         onLoadingStart={this._onLoadingStart}
         onLoadingFinish={this._onLoadingFinish}
     }
 
     this._showRedboxOnPropChanges(prevProps, 'allowsInlineMediaPlayback');
+    this._showRedboxOnPropChanges(prevProps, 'incognito');
     this._showRedboxOnPropChanges(prevProps, 'mediaPlaybackRequiresUserAction');
     this._showRedboxOnPropChanges(prevProps, 'dataDetectorTypes');
 

js/WebViewTypes.js

    */
   source?: ?WebViewSource,
 
+  /**
+   * Does not store any data within the lifetime of the WebView.
+   */
+  incognito?: ?boolean,
+
   /**
    * Function that returns a view to show if there's an error.
    */