Added option to set mixed content mode in android webview

Summary:
PR for option to set mixed content mode in Webview(Android) for issue #8460
Closes https://github.com/facebook/react-native/pull/12314

Differential Revision: D4663084

Pulled By: lacker

fbshipit-source-id: 0e40ea463739166311ddcb7887ff6d0289369637

Libraries/Components/WebView/WebView.ios.js

@@ -340,6 +340,22 @@ class WebView extends React.Component {
      * executed immediately as JavaScript.
      */
     injectJavaScript: PropTypes.func,
+
+    /**
+     * Specifies the mixed content mode. i.e WebView will allow a secure origin to load content from any other origin.
+     *
+     * Possible values for `mixedContentMode` are:
+     *
+     * - `'never'` (default) - WebView will not allow a secure origin to load content from an insecure origin.
+     * - `'always'` - WebView will allow a secure origin to load content from any other origin, even if that origin is insecure.
+     * - `'compatibility'` -  WebView will attempt to be compatible with the approach of a modern web browser with regard to mixed content.
+     * @platform android
+     */
+    mixedContentMode: PropTypes.oneOf([
+      'never',
+      'always',
+      'compatibility'
+    ]),
   };
 
   state = {