Make HandleWatcher watch on the same thread if the thread is running a MessagePumpMojo.

mojo/common/handle_watcher.h

Index: mojo/common/handle_watcher.h
diff --git a/mojo/common/handle_watcher.h b/mojo/common/handle_watcher.h
index f40ae88b42633c2eba97c56dc7ccd0d73a1188d3..dfcb145122f796387ce1be44b1727211b5e04464 100644
--- a/mojo/common/handle_watcher.h
+++ b/mojo/common/handle_watcher.h
@@ -48,10 +48,12 @@ class MOJO_COMMON_EXPORT HandleWatcher {
   void Stop();
 
  private:
-  class State;
+  class StateBase;
+  class SameThreadWatchingState;
+  class SecondaryThreadWatchingState;
 
   // If non-NULL Start() has been invoked.
-  scoped_ptr<State> state_;
+  scoped_ptr<StateBase> state_;
 
   DISALLOW_COPY_AND_ASSIGN(HandleWatcher);
 };