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

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index a979355493f5fcb07eacf0d42c61c182ba783da9..a6616378114a5424766aa8dceda78e086f887d07 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -688,10 +688,8 @@ void RenderWidgetCompositor::setDeviceScaleFactor(float 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) {