[ios] Pass more --jscomp_error switches to the closure Compiler.

ios/web/web_state/js/resources/post_request.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Provides a way to implement POST requests via XMLHttpRequest.
 // Works around https://bugs.webkit.org/show_bug.cgi?id=145410 on WKWebView.
 
 'use strict';
 
 goog.provide('__crPostRequestWorkaround');
@@ skipping 58 matching lines @@
     request.send(blob);
     if (request.status != 200) {
       throw request.status;
     }
     return request.responseText;
   }
 
   document.open();
   try {
     document.write(createAndSendPostRequest(url, headers, body, contentType));
-    window.webkit.messageHandlers.POSTSuccessHandler.postMessage("");
+    window.webkit.messageHandlers['POSTSuccessHandler'].postMessage("");

[dpapad, 2016/10/26 17:05:24]

Can you add externs definitions for those? Then you don't need to bypass the
compiler by using quoted notation.

[Eugene But (OOO till 7-30), 2016/10/26 17:54:45]

I would prefer not to. messageHandlers is an object which is extended in native
code and using dot syntax for it does not feel right in the first place. If
someone adds a new handler they should not be forces to change the code in 3
places (one JS file, Native code, externs file).

[dpapad, 2016/10/26 18:18:35]

It is a tradeoff between inconvenience and better type coverage VS more
flexibility and less type coverage. If programmer makes a typo let's say,
window.webkit.messageHandlers['POSTSucessHandler'].postMessage(), you would get
a runtime error, instead of a compile time error.

Also note that when Closure compiler does not know the type of an object, it is
extremely lenient. The following would not be an error

window.webkit.messageHandlers['POSTSuccessHandler'].foo();
as long as there is any type known to Closure compiler that has a 'foo' method.

[Eugene But (OOO till 7-30), 2016/10/26 20:58:53]

Acknowledged.

   } catch(error) {
-    window.webkit.messageHandlers.POSTErrorHandler.postMessage(error);
+    window.webkit.messageHandlers['POSTErrorHandler'].postMessage(error);
   }
   document.close();
 }