[blink] Support (semi-)transparent background colors in WebView/Frame.

content/renderer/gpu/render_widget_compositor.cc

 // Copyright (c) 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 "content/renderer/gpu/render_widget_compositor.h"
 
 #include <stddef.h>
 
 #include <cmath>
 #include <limits>
@@ skipping 670 matching lines @@
     const WebFloatPoint& point) const {
   return point;
 }
 
 void RenderWidgetCompositor::setDeviceScaleFactor(float device_scale) {
   layer_tree_host_->SetDeviceScaleFactor(device_scale);
 }
 
 void RenderWidgetCompositor::setBackgroundColor(blink::WebColor color) {
   layer_tree_host_->set_background_color(color);
-}
-
-void RenderWidgetCompositor::setHasTransparentBackground(bool transparent) {
-  layer_tree_host_->set_has_transparent_background(transparent);
+  layer_tree_host_->set_has_transparent_background(SkColorGetA(color) <
+                                                   SK_AlphaOPAQUE);
 }
 
 void RenderWidgetCompositor::setVisible(bool visible) {
   if (never_visible_)
     return;
 
   layer_tree_host_->SetVisible(visible);
 }
 
 void RenderWidgetCompositor::setPageScaleFactorAndLimits(
@@ skipping 447 matching lines @@
 void RenderWidgetCompositor::SetContentSourceId(uint32_t id) {
   layer_tree_host_->SetContentSourceId(id);
 }
 
 void RenderWidgetCompositor::SetLocalSurfaceId(
     const cc::LocalSurfaceId& local_surface_id) {
   layer_tree_host_->SetLocalSurfaceId(local_surface_id);
 }
 
 }  // namespace content