[ios] Handle gracefully WK messages sent by iframes.

ios/web/web_state/ui/crw_web_controller.mm

 // Copyright 2012 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.
 
 #import "ios/web/web_state/ui/crw_web_controller.h"
 
 #import <WebKit/WebKit.h>
 
 #import <objc/runtime.h>
 #include <stddef.h>
@@ skipping 2667 matching lines @@
   }
 }
 
 - (NSString*)scriptByAddingWindowIDCheckForScript:(NSString*)script {
   NSString* kTemplate = @"if (__gCrWeb['windowId'] === '%@') { %@; }";
   return [NSString
       stringWithFormat:kTemplate, [_windowIDJSManager windowID], script];
 }
 
 - (BOOL)respondToWKScriptMessage:(WKScriptMessage*)scriptMessage {
-  CHECK(scriptMessage.frameInfo.mainFrame);
+  if (!scriptMessage.frameInfo.mainFrame) {
+    // Messages from iframes are not supported yet.

[Jackie Quinn, 2016/11/23 19:17:40]

super nit: s/not supported yet/not currently supported. Reduces the expectation
that it will be supported in some particular timeframe. If there is a timeline
for support, maybe add a TODO and bug.

[Eugene But (OOO till 7-30), 2016/11/23 19:28:39]

Updated comment. The future of iframe injections is unclear, so I'd prefer to
avoid TODO :).

+    return NO;
+  }
+
   int errorCode = 0;
   std::string errorMessage;
   std::unique_ptr<base::Value> inputJSONData(
       base::JSONReader::ReadAndReturnError(
           base::SysNSStringToUTF8(scriptMessage.body), false, &errorCode,
           &errorMessage));
   if (errorCode) {
     DLOG(WARNING) << "JSON parse error: %s" << errorMessage.c_str();
     return NO;
   }
@@ skipping 3065 matching lines @@
   }
 
   return web::WEB_VIEW_DOCUMENT_TYPE_GENERIC;
 }
 
 - (NSString*)refererFromNavigationAction:(WKNavigationAction*)action {
   return [action.request valueForHTTPHeaderField:@"Referer"];
 }
 
 @end