Let IPC::SyncMessageFilter take advantage of thread-safe Send.

ipc/ipc_channel_proxy.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IPC_IPC_CHANNEL_PROXY_H_
 #define IPC_IPC_CHANNEL_PROXY_H_
 
 #include <vector>
 
 #include "base/memory/ref_counted.h"
@@ skipping 169 matching lines @@
       return ipc_task_runner_.get();
     }
     const std::string& channel_id() const { return channel_id_; }
 
     // Dispatches a message on the listener thread.
     void OnDispatchMessage(const Message& message);
 
     // Sends |message| from appropriate thread.
     void Send(Message* message);
 
+    // Indicates if the underlying channel's Send is thread-safe.
+    bool IsChannelSendThreadSafe() const;
+
    protected:
     friend class base::RefCountedThreadSafe<Context>;
     ~Context() override;
 
     // IPC::Listener methods:
     bool OnMessageReceived(const Message& message) override;
     void OnChannelConnected(int32 peer_pid) override;
     void OnChannelError() override;
 
     // Like OnMessageReceived but doesn't try the filters.
@@ skipping 98 matching lines @@
   bool did_init_;
 
 #if defined(ENABLE_IPC_FUZZER)
   OutgoingMessageFilter* outgoing_message_filter_;
 #endif
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_CHANNEL_PROXY_H_