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

ppapi/shared_impl/tracked_callback.h

Index: ppapi/shared_impl/tracked_callback.h
diff --git a/ppapi/shared_impl/tracked_callback.h b/ppapi/shared_impl/tracked_callback.h
index 92af19bed3500db6738479d2d48531efeabaf2eb..c8b6c01362801df1b4a4c494fd70cca30af79995 100644
--- a/ppapi/shared_impl/tracked_callback.h
+++ b/ppapi/shared_impl/tracked_callback.h
@@ -113,6 +113,10 @@ class PPAPI_SHARED_EXPORT TrackedCallback
     return !callback_.func;
   }
 
+  MessageLoopShared* target_loop() const {
+    return target_loop_.get();
+  }
+
   // Determines if the given callback is pending. A callback is pending if it
   // has not completed and has not been aborted. When receiving a plugin call,
   // use this to detect if |callback| represents an operation in progress. When