Add app.window.alphaEnabled() and onAlphaEnabledChanged.

chrome/browser/ui/cocoa/apps/native_app_window_cocoa.mm

 // Copyright 2013 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.
 
 #include "chrome/browser/ui/cocoa/apps/native_app_window_cocoa.h"
 
 #include "apps/app_shim/extension_app_shim_handler_mac.h"
 #include "base/command_line.h"
 #include "base/mac/foundation_util.h"
 #include "base/mac/mac_util.h"
@@ skipping 781 matching lines @@
   gfx::Rect frame_rect(NSRectToCGRect(frame_nsrect));
   frame_rect.set_y(screen_height - NSMaxY(frame_nsrect));
 
   NSRect content_nsrect = [window() contentRectForFrameRect:frame_nsrect];
   gfx::Rect content_rect(NSRectToCGRect(content_nsrect));
   content_rect.set_y(screen_height - NSMaxY(content_nsrect));
 
   return frame_rect.InsetsFrom(content_rect);
 }
 
+bool NativeAppWindowCocoa::CanHaveTransparentBackground() const {
+  return false;
+}
+
 gfx::NativeView NativeAppWindowCocoa::GetHostView() const {
   NOTIMPLEMENTED();
   return NULL;
 }
 
 gfx::Point NativeAppWindowCocoa::GetDialogPosition(const gfx::Size& size) {
   NOTIMPLEMENTED();
   return gfx::Point();
 }
 
@@ skipping 189 matching lines @@
 }
 
 void NativeAppWindowCocoa::UpdateRestoredBounds() {
   if (IsRestored(*this))
     restored_bounds_ = [window() frame];
 }
 
 void NativeAppWindowCocoa::HideWithoutMarkingHidden() {
   [window() orderOut:window_controller_];
 }