[ios] Protect JS messaging system from window.webkit object overriding.

ios/web/web_state/js/resources/message.js

Index: ios/web/web_state/js/resources/message.js
diff --git a/ios/web/web_state/js/resources/message.js b/ios/web/web_state/js/resources/message.js
index e57133a79efc648d99a59fdf21ac548c5d3da7df..c24213d13487c299a0ef60f6520800b1de80d95a 100644
--- a/ios/web/web_state/js/resources/message.js
+++ b/ios/web/web_state/js/resources/message.js
@@ -116,8 +116,13 @@ __gCrWeb.message = {};
             "crwCommand": command,
             "crwWindowId": __gCrWeb.windowId
         });
+        // If |window.webkit| has been overridden deleting the object restores

[Jackie Quinn, 2016/05/12 17:33:58]

Well that's nifty! Probably still good to have a comment in there explaining why
we need to use the original rather than a potential override.

[Eugene But (OOO till 7-30), 2016/05/12 18:44:41]

I did my best changing the comment :)

+        // the original implementation.
+        var oldWebkit = window.webkit;
+        delete window.webkit;
         window.webkit.messageHandlers[queueObject.scheme].postMessage(
             stringifiedMessage);
+        window.webkit = oldWebkit;
     });
     queueObject.reset();