Add app.window.alphaEnabled() and onAlphaEnabledChanged.

apps/ui/native_app_window.h

Index: apps/ui/native_app_window.h
diff --git a/apps/ui/native_app_window.h b/apps/ui/native_app_window.h
index cde4c1dfad8102de7ce4b484e60c1fcc30e9e5b5..96db494d73b21f4a8ab3dbd893d8fc445dbf9d67 100644
--- a/apps/ui/native_app_window.h
+++ b/apps/ui/native_app_window.h
@@ -87,6 +87,8 @@ class NativeAppWindow : public ui::BaseWindow,
   virtual void SetContentSizeConstraints(const gfx::Size& min_size,
                                          const gfx::Size& max_size) = 0;
 
+  virtual bool CanHaveTransparentBackground() const = 0;

[Wez, 2014/07/09 22:03:29]

Do we want this to be called "transparent background", if the API is becoming
"alphaEnabled"?

[Wez, 2014/07/09 22:03:29]

Add comment e.g. "Returns false if the NativeAppWindow ignores alpha
transparency."

[jackhou1, 2014/07/10 03:04:17]

Oops, done.

[jackhou1, 2014/07/10 03:04:18]

Done.

This CL still conflicts a fair bit with
https://codereview.chromium.org/190533006

I'll probably have to rebase, so I'll go over these names again at that point.

[Wez, 2014/07/11 17:14:10]

Acknowledged.

+
   virtual ~NativeAppWindow() {}
 };