Eliminate a potential race in IPC::ChannelProxy

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 dca8c977302975e5fe9a0bf8c7dc188004d97121..b0247c4d3c9a42fb8924c80e59059e6c258921a8 100644
--- a/ipc/ipc_channel_proxy.h
+++ b/ipc/ipc_channel_proxy.h
@@ -233,6 +233,10 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     // List of filters.  This is only accessed on the IPC thread.
     std::vector<scoped_refptr<MessageFilter> > filters_;
     scoped_refptr<base::SingleThreadTaskRunner> ipc_task_runner_;
+
+    // Note, channel_ may be set on the Listener thread or the IPC thread.
+    // But once it has been set, it must only be read or cleared on the IPC
+    // thread.
     scoped_ptr<Channel> channel_;
     std::string channel_id_;
     bool channel_connected_called_;