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

content/browser/browser_message_filter.h

Index: content/browser/browser_message_filter.h
===================================================================
--- content/browser/browser_message_filter.h	(revision 79364)
+++ content/browser/browser_message_filter.h	(working copy)
@@ -48,6 +48,11 @@
   // 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.