[content] Fix background color update handling in RWHVAura.

content/public/browser/render_widget_host_view.h

Index: content/public/browser/render_widget_host_view.h
diff --git a/content/public/browser/render_widget_host_view.h b/content/public/browser/render_widget_host_view.h
index 71c693bf7c3c09c90b6df776d1afe9dff015fbf2..f7166ea99fcae793e466aaad2b3c011e1235d3fc 100644
--- a/content/public/browser/render_widget_host_view.h
+++ b/content/public/browser/render_widget_host_view.h
@@ -128,7 +128,9 @@ class CONTENT_EXPORT RenderWidgetHostView {
   virtual base::string16 GetSelectedText() = 0;
 
   // Subclasses should override this method to set the background color. |color|
-  // could be transparent or opaque.
+  // has to be either SK_ColorTRANSPARENT or opaque. If set to
+  // SK_ColorTRANSPARENT, the renderer's background color will be overridden to
+  // be fully transparent.
   virtual void SetBackgroundColor(SkColor color) = 0;
   // Convenience method to fill the background layer with the default color by
   // calling |SetBackgroundColor|.