Fix retrieval of unthemed scrollbar size.

ui/native_theme/native_theme_win.cc

Index: ui/native_theme/native_theme_win.cc
diff --git a/ui/native_theme/native_theme_win.cc b/ui/native_theme/native_theme_win.cc
index b3d09a49d9ba0871a70f96cde7a591bee43cc2a6..3c26c97d38ef8ff127ffbad5584e269c01fdd747 100644
--- a/ui/native_theme/native_theme_win.cc
+++ b/ui/native_theme/native_theme_win.cc
@@ -254,6 +254,15 @@ gfx::Size NativeThemeWin::GetPartSize(Part part,
         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;