Ensure that BrowserMessageFilter isn't used to process a sync message on the UI thread.

content/browser/browser_message_filter.h

 // Copyright (c) 2010 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.
 
 #ifndef CONTENT_BROWSER_BROWSER_MESSAGE_FILTER_H_
 #define CONTENT_BROWSER_BROWSER_MESSAGE_FILTER_H_
 #pragma once
 
 #include "base/process.h"
 #include "content/browser/browser_thread.h"
@@ skipping 30 matching lines @@
   // Override this to receive messages.
   // Your function will normally be called on the IO thread.  However, if your
   // OverrideThreadForMessage modifies the thread used to dispatch the message,
   // your function will be called on the requested thread.
   virtual bool OnMessageReceived(const IPC::Message& message,
                                  bool* message_was_ok) = 0;
 
   // Can be called on any thread, after OnChannelConnected is called.
   base::ProcessHandle peer_handle() { return peer_handle_; }
 
+  // Checks that the given message can be dispatched on the UI thread, depending
+  // on the platform.  If not, returns false and an error ot the sender.
+  static bool MessageCanBeDispatchedOnUI(const IPC::Message& message,
+                                         IPC::Message::Sender* sender);
+
  protected:
   // Call this if a message couldn't be deserialized.  This kills the renderer.
   // Can be called on any thread.
   virtual void BadMessageReceived();
 
  private:
   // Dispatches a message to the derived class.
   bool DispatchMessage(const IPC::Message& message);
 
   IPC::Channel* channel_;
   base::ProcessHandle peer_handle_;
 };
 
 #endif  // CONTENT_BROWSER_BROWSER_MESSAGE_FILTER_H_