Add support for transparent background API

apps/app_window.h

Index: apps/app_window.h
diff --git a/apps/app_window.h b/apps/app_window.h
index 3fd57570cf43126dd61ab71ef32c5a8e247a482c..3fd65e81b43a773182fa44a91f62b0c87a483d0d 100644
--- a/apps/app_window.h
+++ b/apps/app_window.h
@@ -144,7 +144,7 @@ class AppWindow : public content::NotificationObserver,
     bool has_frame_color;
     SkColor active_frame_color;
     SkColor inactive_frame_color;
-    bool transparent_background;  // Only supported on ash.
+    bool alpha_enabled;  // Only supported on ash.
 
     // The initial content/inner bounds specification (excluding any window
     // decorations).
@@ -350,6 +350,12 @@ class AppWindow : public content::NotificationObserver,
   // the renderer.
   void GetSerializedState(base::DictionaryValue* properties) const;
 
+  // Whether this window allows a transparent background image.
+  // Currently, only Ash (ChromeOS/Windows) support transparent windows.
+  // This will always return false on platforms that don't support transparent
+  // windows.
+  bool AlphaEnabled() const;
+
  protected:
   virtual ~AppWindow();
 
@@ -528,6 +534,9 @@ class AppWindow : public content::NotificationObserver,
   // taskbar.
   bool cached_always_on_top_;
 
+  // Cache the alpha_enabled attribute from the window creation.
+  bool cached_alpha_enabled_;
+
   DISALLOW_COPY_AND_ASSIGN(AppWindow);
 };