Allow to have a transparent UA dependent background.

ui/views/controls/webview/webview.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_VIEWS_CONTROLS_WEBVIEW_WEBVIEW_H_
 #define UI_VIEWS_CONTROLS_WEBVIEW_WEBVIEW_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
@@ skipping 97 matching lines @@
   void OnBoundsChanged(const gfx::Rect& previous_bounds) override;
   void ViewHierarchyChanged(
       const ViewHierarchyChangedDetails& details) override;
   bool SkipDefaultKeyEventProcessing(const ui::KeyEvent& event) override;
   bool OnMousePressed(const ui::MouseEvent& event) override;
   void OnFocus() override;
   void AboutToRequestFocusFromTabTraversal(bool reverse) override;
   void GetAccessibleState(ui::AXViewState* state) override;
   gfx::NativeViewAccessible GetNativeViewAccessible() override;
   gfx::Size GetPreferredSize() const override;
+  void OnPaintBackground(gfx::Canvas* canvas);
 
   // Overridden from content::RenderProcessHostObserver:
   void RenderProcessExited(content::RenderProcessHost* host,
                            base::TerminationStatus status,
                            int exit_code) override;
   void RenderProcessHostDestroyed(content::RenderProcessHost* host) override;
 
   // Overridden from content::WebContentsDelegate:
   bool EmbedsFullscreenWidget() const override;
 
@@ skipping 46 matching lines @@
   content::BrowserContext* browser_context_;
   bool allow_accelerators_;
   gfx::Size preferred_size_;
 
   DISALLOW_COPY_AND_ASSIGN(WebView);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_WEBVIEW_WEBVIEW_H_