Fix scrollbar transparency / opacity issues on Windows.

ui/base/native_theme/native_theme_win.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_BASE_NATIVE_THEME_NATIVE_THEME_WIN_H_
 #define UI_BASE_NATIVE_THEME_NATIVE_THEME_WIN_H_
 #pragma once
 
 // A wrapper class for working with custom XP/Vista themes provided in
 // uxtheme.dll.  This is a singleton class that can be grabbed using
@@ skipping 94 matching lines @@
   virtual gfx::Size GetPartSize(Part part,
                                 State state,
                                 const ExtraParams& extra) const OVERRIDE;
   virtual void Paint(SkCanvas* canvas,
                      Part part,
                      State state,
                      const gfx::Rect& rect,
                      const ExtraParams& extra) const OVERRIDE;
   virtual SkColor GetSystemColor(ColorId color_id) const OVERRIDE;
 
-  void PaintToNonPlatformCanvas(SkCanvas* canvas,
-                                Part part,
-                                State state,
-                                const gfx::Rect& rect,
-                                const ExtraParams& extra) const;
+  // Paint directly to canvas' HDC.
+  void PaintDirect(SkCanvas* canvas,
+                   Part part,
+                   State state,
+                   const gfx::Rect& rect,
+                   const ExtraParams& extra) const;
+
+  // Create a temporary HDC, paint to that, clean up the alpha values in the
+  // temporary HDC, and then blit the result to canvas.  This is to work around
+  // the fact that Windows XP and some classic themes give bogus alpha values.
+  void PaintIndirect(SkCanvas* canvas,
+                     Part part,
+                     State state,
+                     const gfx::Rect& rect,
+                     const ExtraParams& extra) const;
 
   HRESULT GetThemePartSize(ThemeName themeName,
                            HDC hdc,
                            int part_id,
                            int state_id,
                            RECT* rect,
                            int ts,
                            SIZE* size) const;
 
   HRESULT PaintButton(HDC hdc,
@@ skipping 192 matching lines @@
 
   // A cache of open theme handles.
   mutable HANDLE theme_handles_[LAST];
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeWin);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_NATIVE_THEME_NATIVE_THEME_WIN_H_