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

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

Index: ios/web/web_state/js/resources/post_request.js
diff --git a/ios/web/web_state/js/resources/post_request.js b/ios/web/web_state/js/resources/post_request.js
index 7f5ff25168fb5eb6528ac66f7de57c6df1b194a8..65afcf490fb6377b7349d5dc34478a440202f776 100644
--- a/ios/web/web_state/js/resources/post_request.js
+++ b/ios/web/web_state/js/resources/post_request.js
@@ -76,9 +76,9 @@ __crPostRequestWorkaround.runPostRequest = function(
   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();
 }