Add app.window.alphaEnabled() and onAlphaEnabledChanged.

apps/app_window.h

Index: apps/app_window.h
diff --git a/apps/app_window.h b/apps/app_window.h
index f2e0e98d815be3c42a10bc939588a7e3cf0d1307..52e2b65e73ed79ad8435eabaf3e4ad1be94227e6 100644
--- a/apps/app_window.h
+++ b/apps/app_window.h
@@ -362,6 +362,9 @@ class AppWindow : public content::NotificationObserver,
   // app.
   void WindowEventsReady();
 
+  // Whether the app window wants a transparent background.
+  bool RequestedTransparentBackground() const;
+
  protected:
   virtual ~AppWindow();
 
@@ -558,6 +561,9 @@ class AppWindow : public content::NotificationObserver,
   // taskbar.
   bool cached_always_on_top_;
 
+  // Whether |transparent_background| was set in the CreateParams.
+  bool requested_transparent_background_;
+
   DISALLOW_COPY_AND_ASSIGN(AppWindow);
 };