[ios] Use square brackets syntax for nulling out toJSON property.

Closure reports an error for the old code:

../../ios/web/web_state/js/resources/common.js:40: ERROR - assignment to
property toJSON of __gCrWeb.common.JSONSafeObject.prototype
found   : null
required: function (this:Object, string=): *
  __gCrWeb.common.JSONSafeObject.prototype.toJSON = null;
  ^

BUG=None

Review-Url: https://codereview.chromium.org/2475643003
Cr-Commit-Position: refs/heads/master@{#429702}

ios/web/web_state/js/resources/common.js

@@ -37,7 +37,7 @@ __gCrWeb['common'] = __gCrWeb.common;
   /**
    * Protect against custom implementation of Object.toJSON in host pages.
    */
-  __gCrWeb.common.JSONSafeObject.prototype.toJSON = null;
+  __gCrWeb.common.JSONSafeObject.prototype['toJSON'] = null;
 
   /**
    * Retain the original JSON.stringify method where possible to reduce the