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

ppapi/shared_impl/ppb_message_loop_shared.h

Index: ppapi/shared_impl/ppb_message_loop_shared.h
diff --git a/ppapi/shared_impl/ppb_message_loop_shared.h b/ppapi/shared_impl/ppb_message_loop_shared.h
index fd6d1618d1d4037d2392331c6f6414c46e5a3573..ed7bddb6de0e7675c4b4cb4c81ca645123a9c260 100644
--- a/ppapi/shared_impl/ppb_message_loop_shared.h
+++ b/ppapi/shared_impl/ppb_message_loop_shared.h
@@ -13,6 +13,10 @@
 #include "ppapi/shared_impl/resource.h"
 #include "ppapi/thunk/ppb_message_loop_api.h"
 
+namespace base {
+class MessageLoopProxy;
+}
+
 namespace tracked_objects {
 class Location;
 }
@@ -44,6 +48,8 @@ class PPAPI_SHARED_EXPORT MessageLoopShared
                            const base::Closure& closure,
                            int64 delay_ms) = 0;
 
+  virtual base::MessageLoopProxy* GetMessageLoopProxy() = 0;
+
   DISALLOW_COPY_AND_ASSIGN(MessageLoopShared);
 };