fix(ios): Make allowFileAccessFromFileURLs work in iOS. (#1061)

docs/Reference.md

 
 ### `javaScriptEnabled`
 
-Boolean value to enable JavaScript in the `WebView`. Used on Android only as JavaScript is enabled by default on iOS. The default value is `true`.
+Boolean value to enable JavaScript in the `WebView`. The default value is `true`.
 
-| Type | Required | Platform |
-| ---- | -------- | -------- |
-| bool | No       | Android  |
+| Type | Required |
+| ---- | -------- |
+| bool | No       |
 
 ---
 
 
 Boolean that sets whether JavaScript running in the context of a file scheme URL should be allowed to access content from other file scheme URLs. The default value is `false`.
 
-| Type | Required | Platform |
-| ---- | -------- | -------- |
-| bool | No       | Android  |
+| Type | Required |
+| ---- | -------- |
+| bool | No       |
 
 ---
 

ios/RNCWebView.h

 @property (nonatomic, copy) NSString * _Nullable applicationNameForUserAgent;
 @property (nonatomic, assign) BOOL cacheEnabled;
 @property (nonatomic, assign) BOOL javaScriptEnabled;
+@property (nonatomic, assign) BOOL allowFileAccessFromFileURLs;
 @property (nonatomic, assign) BOOL allowsLinkPreview;
 @property (nonatomic, assign) BOOL showsHorizontalScrollIndicator;
 @property (nonatomic, assign) BOOL showsVerticalScrollIndicator;

ios/RNCWebView.m

   if (self.window != nil && _webView == nil) {
     WKWebViewConfiguration *wkWebViewConfig = [WKWebViewConfiguration new];
     WKPreferences *prefs = [[WKPreferences alloc]init];
+    BOOL _prefsUsed = NO;
     if (!_javaScriptEnabled) {
       prefs.javaScriptEnabled = NO;
+      _prefsUsed = YES;
+    }
+    if (_allowFileAccessFromFileURLs) {
+      [prefs setValue:@TRUE forKey:@"allowFileAccessFromFileURLs"];
+      _prefsUsed = YES;
+    }
+    if (_prefsUsed) {
       wkWebViewConfig.preferences = prefs;
     }
     if (_incognito) {

ios/RNCWebViewManager.m

 RCT_EXPORT_VIEW_PROPERTY(injectedJavaScript, NSString)
 RCT_EXPORT_VIEW_PROPERTY(injectedJavaScriptBeforeContentLoaded, NSString)
 RCT_EXPORT_VIEW_PROPERTY(javaScriptEnabled, BOOL)
+RCT_EXPORT_VIEW_PROPERTY(allowFileAccessFromFileURLs, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(allowsInlineMediaPlayback, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(mediaPlaybackRequiresUserAction, BOOL)
 #if WEBKIT_IOS_10_APIS_AVAILABLE