Allow to have a transparent UA dependent background.

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 b5f413fc94afa96a6cc1a2884c1c85fa9940efe9..2f7def8a50288c368ae19bd7070b628e9d2848a9 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.cc
+++ b/content/browser/renderer_host/render_widget_host_view_aura.cc
@@ -809,6 +809,11 @@ void RenderWidgetHostViewAura::Focus() {
   aura::client::FocusClient* client = aura::client::GetFocusClient(window_);
   if (client)
     window_->Focus();
+
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kTransparentBackgroundRWHV)) {
+    SetBackgroundColor(SK_ColorTRANSPARENT);
+  }
 }
 
 bool RenderWidgetHostViewAura::HasFocus() const {