Allow proxy channels to be created without initializing the underlying channel.

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 29844da5629a5d4ade4e5c7f4d65536b0ad95862..6100c11d4c26804524f7b45b8d08b4a488ac0f44 100644
--- a/ipc/ipc_sync_channel.h
+++ b/ipc/ipc_sync_channel.h
@@ -63,12 +63,22 @@ class MessageReplyDeserializer;
 class IPC_EXPORT SyncChannel : public ChannelProxy,
                                public base::WaitableEventWatcher::Delegate {
  public:
+  // Creates and initializes a sync channel. If create_pipe_now is specified,
+  // the channel will be initialized synchronously.
   SyncChannel(const IPC::ChannelHandle& channel_handle,
               Channel::Mode mode,
               Channel::Listener* listener,
               base::MessageLoopProxy* ipc_message_loop,
               bool create_pipe_now,
               base::WaitableEvent* shutdown_event);
+
+  // Creates an uninitialized sync channel. Call ChannelProxy::Init to
+  // initialize the channel. This two-step setup allows message filters to be
+  // added before any messages are sent or received.
+  SyncChannel(Channel::Listener* listener,
+              base::MessageLoopProxy* ipc_message_loop,
+              base::WaitableEvent* shutdown_event);
+
   virtual ~SyncChannel();
 
   virtual bool Send(Message* message);