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

ppapi/proxy/ppb_message_loop_proxy.cc

Index: ppapi/proxy/ppb_message_loop_proxy.cc
diff --git a/ppapi/proxy/ppb_message_loop_proxy.cc b/ppapi/proxy/ppb_message_loop_proxy.cc
index 7e2cdfa65593cbe51b7ba28a146fa84400c6c7fd..582ddcbeb73d92e3fe869ba30fbc0d3eb576af83 100644
--- a/ppapi/proxy/ppb_message_loop_proxy.cc
+++ b/ppapi/proxy/ppb_message_loop_proxy.cc
@@ -189,6 +189,10 @@ void MessageLoopResource::PostClosure(
   }
 }
 
+base::MessageLoopProxy* MessageLoopResource::GetMessageLoopProxy() {
+  return loop_proxy_.get();
+}
+
 // static
 void MessageLoopResource::ReleaseMessageLoop(void* value) {
   static_cast<MessageLoopResource*>(value)->DetachFromThread();