Use the main world script context for mime handler postMessage.

extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc

 // Copyright 2014 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.
 
 #include "extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.h"
 
 #include <map>
 #include <set>
 
 #include "content/public/renderer/render_frame.h"
@@ skipping 185 matching lines @@
 
   content::RenderView* guest_proxy_render_view =
       content::RenderView::FromRoutingID(guest_proxy_routing_id_);
   if (!guest_proxy_render_view)
     return;
   blink::WebFrame* guest_proxy_frame =
       guest_proxy_render_view->GetWebView()->mainFrame();
   if (!guest_proxy_frame)
     return;
 
+  v8::Context::Scope context_scope(
+      render_frame()->GetWebFrame()->mainWorldScriptContext());
   v8::Local<v8::Object> guest_proxy_window =
       guest_proxy_frame->mainWorldScriptContext()->Global();
   gin::Dictionary window_object(isolate, guest_proxy_window);
   v8::Handle<v8::Function> post_message;
   if (!window_object.Get(std::string(kPostMessageName), &post_message))
     return;
 
   v8::Handle<v8::Value> args[] = {
     message,
     // Post the message to any domain inside the browser plugin. The embedder
@@ skipping 71 matching lines @@
 
   if (!render_frame())
     return;
 
   render_frame()->Send(new ExtensionHostMsg_CreateMimeHandlerViewGuest(
       render_frame()->GetRoutingID(), view_id_, element_instance_id(),
       element_size_));
 }
 
 }  // namespace extensions