ChannelProxy: Send() from UI thread when ChannelMojo is used.

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 9ef8bf35f27e7786f7540874f2ee0e38dae71df9..e9d0270a7e3f0f02e515af9a33fff1b32fc52f22 100644
--- a/ipc/ipc_channel_proxy.h
+++ b/ipc/ipc_channel_proxy.h
@@ -55,6 +55,16 @@ class SendCallbackHelper;
 // The consumer of IPC::ChannelProxy is responsible for allocating the Thread
 // instance where the IPC::Channel will be created and operated.
 //
+// Thread-safe send
+//
+// A particular |Channel| implementation has thread-safe |Send()|
+// implementation. If it is supported, ChannelProxy bypasses inter-thread
+// messaging and calls |Send()| immediately. The |channel_| variable is touched
+// from multiple threads in this case, so |ChannelProxy::channel_lifetime_lock_|
+// is used to protect it.
+// This lock overhead is paid only if the underlying channel supports the
+// thread-safe send.
+//
 class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
  public:
   // Initializes a channel proxy.  The channel_handle and mode parameters are
@@ -193,6 +203,11 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     void OnDispatchError();
     void OnDispatchBadMessage(const Message& message);
 
+    // Sends |message| from appropriate thread.
+    void Send(Message* message);
+    void SendFromThisThread(Message* message);
+    void ClearChannel();
+
     scoped_refptr<base::SingleThreadTaskRunner> listener_task_runner_;
     Listener* listener_;
 
@@ -203,6 +218,7 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     // 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.
+    // Here is one exception, which is thread-safe send. See the class comment.
     scoped_ptr<Channel> channel_;
     std::string channel_id_;
     bool channel_connected_called_;
@@ -217,6 +233,13 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     // Lock for pending_filters_.
     base::Lock pending_filters_lock_;
 
+    // Lock for |channel_| value. This is only relevant in the context of
+    // thread-safe send.
+    base::Lock channel_lifetime_lock_;
+    // Indicates the thread-safe send availability. This is constant once
+    // |channel_| is set.
+    bool channel_send_thread_safe_;
+
     // Cached copy of the peer process ID. Set on IPC but read on both IPC and
     // listener threads.
     base::ProcessId peer_pid_;