[Aura] Support transparent webkit.

content/browser/renderer_host/render_widget_host_view_aura.cc

 // Copyright (c) 2011 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 "content/browser/renderer_host/render_widget_host_view_aura.h"
 
 #include "base/logging.h"
 #include "content/browser/renderer_host/backing_store_skia.h"
 #include "content/browser/renderer_host/web_input_event_aura.h"
 #include "content/browser/renderer_host/render_widget_host.h"
@@ skipping 219 matching lines @@
 }
 
 void RenderWidgetHostViewAura::AcceleratedSurfaceRelease(uint64 surface_id) {
   accelerated_surface_containers_.erase(surface_id);
 }
 #endif
 
 void RenderWidgetHostViewAura::SetBackground(const SkBitmap& background) {
   RenderWidgetHostView::SetBackground(background);
   host_->SetBackground(background);
+  window_->layer()->SetFillsBoundsOpaquely(background.isOpaque());
 }
 
 #if defined(OS_POSIX)
 void RenderWidgetHostViewAura::GetDefaultScreenInfo(
     WebKit::WebScreenInfo* results) {
   NOTIMPLEMENTED();
 }
 
 void RenderWidgetHostViewAura::GetScreenInfo(WebKit::WebScreenInfo* results) {
   NOTIMPLEMENTED();
@@ skipping 165 matching lines @@
 
 #if !defined(TOUCH_UI)
 ////////////////////////////////////////////////////////////////////////////////
 // RenderWidgetHostViewAura, private:
 
 void RenderWidgetHostViewAura::UpdateCursorIfOverSelf() {
   //NOTIMPLEMENTED();
   // TODO(beng): See RenderWidgetHostViewWin.
 }
 #endif