Allow to have a transparent UA dependent background.

ui/views/controls/webview/webview.cc

Index: ui/views/controls/webview/webview.cc
diff --git a/ui/views/controls/webview/webview.cc b/ui/views/controls/webview/webview.cc
index 44baa228dc32c69c48a85537ce4c832f9d49f37b..44beb47507db6bca208442224c4b8011854434a5 100644
--- a/ui/views/controls/webview/webview.cc
+++ b/ui/views/controls/webview/webview.cc
@@ -400,4 +400,11 @@ content::WebContents* WebView::CreateWebContents(
   return contents;
 }
 
+void WebView::OnPaintBackground(gfx::Canvas* canvas) {
+  if (!GetWebContents()->GetRenderWidgetHostView()->GetBackgroundOpaque())
+    return;
+
+  View::OnPaintBackground(canvas);
+}
+
 }  // namespace views