Fix retrieval of unthemed scrollbar size.

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 236 matching lines @@
     // when theming is not enabled.  Support for other parts can be added
     // if/when needed.
     switch (part) {
       case kCheckbox:
       case kRadio:
         // TODO(rogerta): I was not able to find any API to get the default
         // size of these controls, so determined these values empirically.
         size.cx = 13;
         size.cy = 13;
         break;
+      case kScrollbarDownArrow:
+      case kScrollbarLeftArrow:
+      case kScrollbarRightArrow:
+      case kScrollbarUpArrow:
+      case kScrollbarHorizontalThumb:
+      case kScrollbarVerticalThumb:
+        size.cx = size.cy = static_cast<int>(GetSystemMetrics(SM_CXVSCROLL) /
+          ui::win::GetDeviceScaleFactor());
+        break;
       default:
         size.cx = 0;
         size.cy = 0;
         break;
     }
   }
 
   return gfx::Size(size.cx, size.cy);
 }
 
@@ skipping 1790 matching lines @@
     handle = open_theme_(NULL, L"Spin");
     break;
   default:
     NOTREACHED();
   }
   theme_handles_[theme_name] = handle;
   return handle;
 }
 
 }  // namespace ui