Allow to have a transparent UA dependent background.

content/browser/renderer_host/render_widget_host_view_base.cc

Index: content/browser/renderer_host/render_widget_host_view_base.cc
diff --git a/content/browser/renderer_host/render_widget_host_view_base.cc b/content/browser/renderer_host/render_widget_host_view_base.cc
index 9cdceb518eefad1e7dd47f841d32010a917e8044..58cf7d982d36abc793e8ea0de29e6a10ddf516a2 100644
--- a/content/browser/renderer_host/render_widget_host_view_base.cc
+++ b/content/browser/renderer_host/render_widget_host_view_base.cc
@@ -371,6 +371,7 @@ const int kFlushInputRateInUs = 16666;
 RenderWidgetHostViewBase::RenderWidgetHostViewBase()
     : popup_type_(blink::WebPopupTypeNone),
       background_color_(SK_ColorWHITE),
+      background_opaque_(false),
       mouse_locked_(false),
       showing_context_menu_(false),
       selection_text_offset_(0),
@@ -415,8 +416,12 @@ void RenderWidgetHostViewBase::SetBackgroundColorToDefault() {
   SetBackgroundColor(SK_ColorWHITE);
 }
 
+void RenderWidgetHostViewBase::SetBackgroundOpaque(bool opaque) {
+  background_opaque_ = opaque;
+}
+
 bool RenderWidgetHostViewBase::GetBackgroundOpaque() {
-  return SkColorGetA(background_color_) == SK_AlphaOPAQUE;
+  return background_opaque_;
 }
 
 gfx::Size RenderWidgetHostViewBase::GetPhysicalBackingSize() const {