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

ipc/ipc_channel.h

Index: ipc/ipc_channel.h
diff --git a/ipc/ipc_channel.h b/ipc/ipc_channel.h
index 2fc52cb58750684ffc4aa40e3ff3565c69908675..9dc91357a8dcd897a4517f211de0696704ab9b37 100644
--- a/ipc/ipc_channel.h
+++ b/ipc/ipc_channel.h
@@ -180,6 +180,10 @@ class IPC_EXPORT Channel : public Sender {
   // deleted once the contents of the Message have been sent.
   virtual bool Send(Message* message) override = 0;
 
+  // True for a particular |Channel| subclass that allows non-IO threads call
+  // |Send()|. The return value must be constant across the lifetime.
+  virtual bool IsSendThreadSafe() const;
+
   // NaCl in Non-SFI mode runs on Linux directly, and the following functions
   // compiled on Linux are also needed. Please see also comments in
   // components/nacl_nonsfi.gyp for more details.