Provide a Mojo equivalent of ThreadSafeSender.

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 5765630a2582349a9b470785eef7c4ae6842bd0f..4f458ed46672520216a5429c1d49441118486a76 100644
--- a/ipc/ipc_channel_proxy.h
+++ b/ipc/ipc_channel_proxy.h
@@ -230,6 +230,10 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
   }
 #endif
 
+  using RunOnIOThreadCallback = base::Callback<void(Channel*)>;
+  // Invokes |callback| on the IO (IPC) thread.
+  void RunOnIOThread(const RunOnIOThreadCallback& callback);
+
   // Called to clear the pointer to the IPC task runner when it's going away.
   void ClearIPCTaskRunner();
 
@@ -239,7 +243,6 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
   // to the internal state.
   explicit ChannelProxy(Context* context);
 
-
   // Used internally to hold state that is referenced on the IPC thread.
   class Context : public base::RefCountedThreadSafe<Context>,
                   public Listener {
@@ -304,6 +307,7 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     void OnSendMessage(std::unique_ptr<Message> message_ptr);
     void OnAddFilter();
     void OnRemoveFilter(MessageFilter* filter);
+    void RunWithChannel(const RunOnIOThreadCallback& callback);
 
     // Methods called on the listener thread.
     void AddFilter(MessageFilter* filter);