Support using TrackedCallbacks as hints to determine the handling thread of resource reply messages

ppapi/proxy/ppb_message_loop_proxy.h

Index: ppapi/proxy/ppb_message_loop_proxy.h
diff --git a/ppapi/proxy/ppb_message_loop_proxy.h b/ppapi/proxy/ppb_message_loop_proxy.h
index 4e3a9332286ce448eb82e20e76f1df0619c45827..d8bfc4c644d92fdc3c24769dbb8977969f6b62b9 100644
--- a/ppapi/proxy/ppb_message_loop_proxy.h
+++ b/ppapi/proxy/ppb_message_loop_proxy.h
@@ -63,6 +63,8 @@ class PPAPI_PROXY_EXPORT MessageLoopResource : public MessageLoopShared {
                            const base::Closure& closure,
                            int64 delay_ms) OVERRIDE;
 
+  virtual base::MessageLoopProxy* GetMessageLoopProxy() OVERRIDE;
+
   // TLS destructor function.
   static void ReleaseMessageLoop(void* value);