Make DesktopWindowTreeHostX11::ShouldWindowContentsBeTransparent() relies on argb visual flag

ui/views/test/widget_test_mac.mm

 // Copyright 2014 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 "ui/views/test/widget_test.h"
 
 #include <Cocoa/Cocoa.h>
 
 #import "base/mac/scoped_nsobject.h"
 #import "base/mac/scoped_objc_class_swizzler.h"
@@ skipping 71 matching lines @@
 ui::EventProcessor* WidgetTest::GetEventProcessor(Widget* widget) {
   return static_cast<internal::RootView*>(widget->GetRootView());
 }
 
 // static
 ui::internal::InputMethodDelegate* WidgetTest::GetInputMethodDelegateForWidget(
     Widget* widget) {
   return NativeWidgetMac::GetBridgeForNativeWindow(widget->GetNativeWindow());
 }
 
+// static
+bool WidgetTest::IsNativeWindowTransparent(gfx::NativeWindow window) {
+  return ![window isOpaque];
+}
+
 }  // namespace test
 }  // namespace views