<WebView>: Add propType for injectJavaScript

Summary:
Small fix. Adds a proptype for the injectJavaScript function, added in da9a712a9e.
Closes https://github.com/facebook/react-native/pull/12665

Differential Revision: D4645803

Pulled By: hramos

fbshipit-source-id: 863e5bf82b853914ac08f9f5b2c141d35882afc7

Libraries/Components/WebView/WebView.android.js

@@ -160,6 +160,12 @@ class WebView extends React.Component {
      * @platform android
      */
     allowUniversalAccessFromFileURLs: PropTypes.bool,
+
+    /**
+     * Function that accepts a string that will be passed to the WebView and
+     * executed immediately as JavaScript.
+     */
+    injectJavaScript: PropTypes.func,
   };
 
   static defaultProps = {

Libraries/Components/WebView/WebView.ios.js

@@ -334,6 +334,12 @@ class WebView extends React.Component {
      * to tap them before they start playing. The default value is `true`.
      */
     mediaPlaybackRequiresUserAction: PropTypes.bool,
+
+    /**
+     * Function that accepts a string that will be passed to the WebView and
+     * executed immediately as JavaScript.
+     */
+    injectJavaScript: PropTypes.func,
   };
 
   state = {