Complete SyncMessageFilter initialization after SyncChannel initialization

ipc/ipc_channel_proxy.h

Index: ipc/ipc_channel_proxy.h
diff --git a/ipc/ipc_channel_proxy.h b/ipc/ipc_channel_proxy.h
index f9f7d3166f17183c32dd325bca9fec611a843543..b01ccbc237408fe23cec60e06ee32ea46a28f07b 100644
--- a/ipc/ipc_channel_proxy.h
+++ b/ipc/ipc_channel_proxy.h
@@ -289,9 +289,15 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
   }
 #endif
 
+ protected:
+  bool did_init() const { return did_init_; }
+
  private:
   friend class IpcSecurityTestUtil;
 
+  // Always called once immediately after Init.
+  virtual void OnChannelInit();
+
   // By maintaining this indirection (ref-counted) to our internal state, we
   // can safely be destroyed while the background thread continues to do stuff
   // that involves this data.