Let IPC::SyncMessageFilter take advantage of thread-safe Send.

ipc/ipc_sync_channel.h

Index: ipc/ipc_sync_channel.h
diff --git a/ipc/ipc_sync_channel.h b/ipc/ipc_sync_channel.h
index 8b4b9a96e363e17187ef6f5e4efe07644628e68d..2fd1b117efca76a75db3a773e35d32e0ba9de533 100644
--- a/ipc/ipc_sync_channel.h
+++ b/ipc/ipc_sync_channel.h
@@ -15,6 +15,7 @@
 #include "ipc/ipc_channel_handle.h"
 #include "ipc/ipc_channel_proxy.h"
 #include "ipc/ipc_sync_message.h"
+#include "ipc/ipc_sync_message_filter.h"
 
 namespace base {
 class WaitableEvent;
@@ -116,6 +117,11 @@ class IPC_EXPORT SyncChannel : public ChannelProxy {
   // default) will be dispatched in any case.
   void SetRestrictDispatchChannelGroup(int group);
 
+  // Creates a new IPC::SyncMessageFilter and adds it to this SyncChannel.
+  // This should be used instead of directly constructing a new
+  // SyncMessageFilter.
+  scoped_refptr<IPC::SyncMessageFilter> CreateSyncMessageFilter();
+
  protected:
   class ReceivedSyncMsgQueue;
   friend class ReceivedSyncMsgQueue;