Implemented stopLoading

Summary:**Motivation:** In my app, I'm using a WebView that loads content from my mobile site.  What I want to do is when a user presses a link on the loaded page, I want to stop the WebView's request, hijack the URL and open the URL in a new WebView, pushed to the top of the navigator stack.  To me, this gives the overall app a more native feel, instead of implementing a rudimentary navbar on the main WebView to go back.

**Attempted Workarounds:** I've attempted to get similar functionality by capturing the onNavigationStateChange event in the WebView, and then within calling goBack + pushing the new view to the navigator stack.  From a functionality standpoint, this works.  However, from a UI standpoint, the user can clearly see the webview change states to a new page + go back before having the new view pushed on top of their nav stack.
Closes https://github.com/facebook/react-native/pull/6886

Differential Revision: D3212447

Pulled By: mkonicek

fb-gh-sync-id: 05911e583d9ba54ddbd54a772153c80ed227731e
fbshipit-source-id: 05911e583d9ba54ddbd54a772153c80ed227731e

Libraries/Components/WebView/WebView.android.js

@@ -264,6 +264,14 @@ var WebView = React.createClass({
     );
   },
 
+  stopLoading: function() {
+    UIManager.dispatchViewManagerCommand(
+      this.getWebViewHandle(),
+      UIManager.RCTWebView.Commands.stopLoading,
+      null
+    );
+  },
+
   /**
    * We return an event with a bunch of fields including:
    *  url, title, loading, canGoBack, canGoForward

Libraries/Components/WebView/WebView.ios.js

@@ -362,6 +362,14 @@ var WebView = React.createClass({
     );
   },
 
+  stopLoading: function() {
+    UIManager.dispatchViewManagerCommand(
+      this.getWebViewHandle(),
+      UIManager.RCTWebView.Commands.stopLoading,
+      null
+    );
+  },
+
   /**
    * We return an event with a bunch of fields including:
    *  url, title, loading, canGoBack, canGoForward