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

mojo/common/message_pump_mojo.h

Index: mojo/common/message_pump_mojo.h
diff --git a/mojo/common/message_pump_mojo.h b/mojo/common/message_pump_mojo.h
index 7b2c1703162dbef35cf4b13a8a71412e20b6418d..8b71316bf79d9e0facc09e5232ec5e9344481944 100644
--- a/mojo/common/message_pump_mojo.h
+++ b/mojo/common/message_pump_mojo.h
@@ -29,6 +29,11 @@ class MOJO_COMMON_EXPORT MessagePumpMojo : public base::MessagePump {
   // using |base::Bind()|).
   static scoped_ptr<base::MessagePump> Create();
 
+  // Returns the MessagePumpMojo instance of the current thread, if it exists.
+  static MessagePumpMojo* current();
+
+  static bool IsCurrent() { return !!current(); }
+
   // Registers a MessagePumpMojoHandler for the specified handle. Only one
   // handler can be registered for a specified handle.
   // NOTE: a value of 0 for |deadline| indicates an indefinite timeout.