Implement first part of supporting global extension commands.

ui/gfx/win/singleton_hwnd.h

 // Copyright (c) 2012 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 UI_GFX_WIN_SINGLETON_HWND_H_
 #define UI_GFX_WIN_SINGLETON_HWND_H_
 
 #include <windows.h>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/observer_list.h"
 #include "ui/gfx/win/window_impl.h"
 
 template<typename T> struct DefaultSingletonTraits;
 
 namespace gfx {
 
 // Singleton message-only HWND that allows interested clients to receive WM_*
 // notifications.
-class SingletonHwnd : public WindowImpl {
+class GFX_EXPORT SingletonHwnd : public WindowImpl {
  public:
   static SingletonHwnd* GetInstance();
 
   // Observer interface for receiving Windows WM_* notifications.
   class Observer {
    public:
     virtual void OnWndProc(HWND hwnd,
                            UINT message,
                            WPARAM wparam,
                            LPARAM lparam) = 0;
   };
 
+  // Creates the window used to receive the messages. Also called implicitly by
+  // AddObserver if the window has not been created before then.
+  void Init();

[sky, 2013/10/03 18:01:22]

Is there a way not to expose this? Could we move this code into the constructor?

[Finnur, 2013/10/03 21:57:58]

Sure, we can create the window in the constructor. But it looked to me like we
want to create the window as late as possible, which in my case complicates the
design, so I opted for an optional "create now" function.

If you prefer I can create the window in the constructor. Just say the word.

[sky, 2013/10/03 23:30:10]

The constructor is only run on the first call to GetInstance() right? At that
point creating the window seems good to me. So, yes please:)

[Finnur, 2013/10/04 17:57:02]

Done.

+
   // Add/remove observer to receive WM_* notifications.
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
   // Windows callback for WM_* notifications.
   virtual BOOL ProcessWindowMessage(HWND window,
                                     UINT message,
                                     WPARAM wparam,
                                     LPARAM lparam,
                                     LRESULT& result,
                                     DWORD msg_map_id) OVERRIDE;
 
  private:
   friend struct DefaultSingletonTraits<SingletonHwnd>;
 
   SingletonHwnd();
   ~SingletonHwnd();
 
   // List of registered observers.
   ObserverList<Observer> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(SingletonHwnd);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_WIN_SINGLETON_HWND_H_