[Mojo] Fix the namespace for HandleWrapper

mojo/bindings/js/handle_close_observer.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_BINDINGS_JS_HANDLE_CLOSE_OBSERVER_H_
 #define MOJO_BINDINGS_JS_HANDLE_CLOSE_OBSERVER_H_
 
-namespace gin {
+namespace mojo {
+namespace js {
 
 class HandleCloseObserver {
  public:
   virtual void OnWillCloseHandle() = 0;
 
  protected:
   virtual ~HandleCloseObserver() {}
 };
 
-}  // namespace gin
+}  // namespace js
+}  // namespace mojo
 
 #endif  // MOJO_BINDINGS_JS_HANDLE_CLOSE_OBSERVER_H_