Fix missing scrollbar thumb on WinXP.

ui/native_theme/native_theme_win.cc

 // 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.
 
 #include "ui/native_theme/native_theme_win.h"
 
 #include <windows.h>
 #include <uxtheme.h>
 #include <vsstyle.h>
 #include <vssym32.h>
@@ skipping 292 matching lines @@
         CommonThemePaintMenuItemBackground(canvas, state, rect);
         return;
     }
   }
 
   bool needs_paint_indirect = false;
   if (!skia::SupportsPlatformPaint(canvas)) {
     // This block will only get hit with --enable-accelerated-drawing flag.
     needs_paint_indirect = true;
   } else {
-    // Scrollbars on Windows XP and the Windows Classic theme have particularly
-    // problematic alpha values, so always draw them indirectly.
+    // Scrollbar components on Windows Classic theme (on all Windows versions)
+    // have particularly problematic alpha values, so always draw them
+    // indirectly. In addition, scrollbar thumbs and grippers for the Windows XP
+    // theme (available only on Windows XP) also need their alpha values
+    // fixed.
     switch (part) {
       case kScrollbarDownArrow:
       case kScrollbarUpArrow:
       case kScrollbarLeftArrow:
       case kScrollbarRightArrow:
+        if (!GetThemeHandle(SCROLLBAR))
+          needs_paint_indirect = true;
+        break;
       case kScrollbarHorizontalThumb:
       case kScrollbarVerticalThumb:
       case kScrollbarHorizontalGripper:
       case kScrollbarVerticalGripper:
-        if (!GetThemeHandle(SCROLLBAR))
+        if (!GetThemeHandle(SCROLLBAR) ||
+            base::win::GetVersion() == base::win::VERSION_XP)
           needs_paint_indirect = true;
         break;
       default:
         break;
     }
   }
 
   if (needs_paint_indirect)
     PaintIndirect(canvas, part, state, rect, extra);
   else
@@ skipping 1752 matching lines @@
     handle = open_theme_(NULL, L"Spin");
     break;
   default:
     NOTREACHED();
   }
   theme_handles_[theme_name] = handle;
   return handle;
 }
 
 }  // namespace ui