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

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 6ff5cc2a9e09bec2cd962ecff746dafaaf3c56a9..f9f7d3166f17183c32dd325bca9fec611a843543 100644
--- a/ipc/ipc_channel_proxy.h
+++ b/ipc/ipc_channel_proxy.h
@@ -187,6 +187,9 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     // 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;