[js] add shared `enableCache` property, defaults to true

js/WebView.android.js

     allowFileAccess: false,
     saveFormDataDisabled: false,
     originWhitelist: WebViewShared.defaultOriginWhitelist,
+    enableCache: true,
   };
 
   state = {
     const { onMessage } = this.props;
     onMessage && onMessage(event);
   };
-  
+
   onLoadingProgress = (event: WebViewProgressEvent) => {
     const { onLoadProgress} = this.props;
     onLoadProgress && onLoadProgress(event);

js/WebView.ios.js

 
   static defaultProps = {
     useWebKit: true,
+    enableCache: true,
     originWhitelist: WebViewShared.defaultOriginWhitelist,
   };
 

js/WebViewTypes.js

    */
   nativeConfig?: ?WebViewNativeConfig,
 
+  /**
+   * Should caching be enabled. Default is true.
+   */
+  enableCache?: ?boolean,
+
   style?: ViewStyleProp,
   children: Node,
 |}>;