Allow to have a transparent UA dependent background.

content/browser/renderer_host/render_widget_host_view_base.h

Index: content/browser/renderer_host/render_widget_host_view_base.h
diff --git a/content/browser/renderer_host/render_widget_host_view_base.h b/content/browser/renderer_host/render_widget_host_view_base.h
index 94fbbb63af8992c4c305c64a6f0bc751540dddac..70d78155f9c0252cead0201647d8cd7128a01820 100644
--- a/content/browser/renderer_host/render_widget_host_view_base.h
+++ b/content/browser/renderer_host/render_widget_host_view_base.h
@@ -85,6 +85,7 @@ class CONTENT_EXPORT RenderWidgetHostViewBase : public RenderWidgetHostView,
   // RenderWidgetHostView implementation.
   void SetBackgroundColor(SkColor color) override;
   void SetBackgroundColorToDefault() final;
+  void SetBackgroundOpaque(bool opaque) override;
   bool GetBackgroundOpaque() override;
   ui::TextInputClient* GetTextInputClient() override;
   void WasUnOccluded() override {}
@@ -428,6 +429,9 @@ class CONTENT_EXPORT RenderWidgetHostViewBase : public RenderWidgetHostView,
   // The background color of the web content.
   SkColor background_color_;
 
+  // The background transparency of the web content.
+  bool background_opaque_;
+
   // While the mouse is locked, the cursor is hidden from the user. Mouse events
   // are still generated. However, the position they report is the last known
   // mouse position just as mouse lock was entered; the movement they report