Implement first part of supporting global extension commands.

ui/gfx/win/singleton_hwnd.h

Index: ui/gfx/win/singleton_hwnd.h
diff --git a/ui/gfx/win/singleton_hwnd.h b/ui/gfx/win/singleton_hwnd.h
index 96af6e792056923fddd38467c3c3efbaf9c076a9..e859375801ef98eca344938575f8c90d29db918b 100644
--- a/ui/gfx/win/singleton_hwnd.h
+++ b/ui/gfx/win/singleton_hwnd.h
@@ -19,7 +19,7 @@ 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();
 
@@ -32,6 +32,10 @@ class SingletonHwnd : public WindowImpl {
                            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);