[Aura] Support transparent webkit.

content/browser/renderer_host/render_widget_host_view_aura.cc

Index: content/browser/renderer_host/render_widget_host_view_aura.cc
diff --git a/content/browser/renderer_host/render_widget_host_view_aura.cc b/content/browser/renderer_host/render_widget_host_view_aura.cc
index 299368f82f28d7f558803c8a7085a65d9aa33739..f4e56b1ecb00870f904a5ec2148fd1cb4ea1a416 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.cc
+++ b/content/browser/renderer_host/render_widget_host_view_aura.cc
@@ -15,13 +15,13 @@
 #include "ui/aura/hit_test.h"
 #include "ui/aura/window.h"
 #include "ui/gfx/canvas.h"
+#include "ui/gfx/compositor/layer.h"
 
 #if defined(UI_COMPOSITOR_IMAGE_TRANSPORT)
 #include "base/bind.h"
 #include "content/browser/gpu/gpu_process_host_ui_shim.h"
 #include "content/browser/renderer_host/accelerated_surface_container_linux.h"
 #include "content/common/gpu/gpu_messages.h"
-#include "ui/gfx/compositor/layer.h"
 #include "ui/gfx/gl/gl_bindings.h"
 #endif
 
@@ -237,6 +237,7 @@ void RenderWidgetHostViewAura::AcceleratedSurfaceRelease(uint64 surface_id) {
 void RenderWidgetHostViewAura::SetBackground(const SkBitmap& background) {
   RenderWidgetHostView::SetBackground(background);
   host_->SetBackground(background);
+  window_->layer()->SetFillsBoundsOpaquely(background.isOpaque());
 }
 
 #if defined(OS_POSIX)