Fix scrollbar buttons at hidpi when enable-use-zoom-for-dsf is on.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp

 /*
  * Copyright (c) 2008, 2009, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 308 matching lines @@
     for (ScrollbarPart part : kButtonParts) {
         if (buttonPartPaintingParams(scrollbar, oldPosition, part) != buttonPartPaintingParams(scrollbar, newPosition, part))
             invalidParts = static_cast<ScrollbarPart>(invalidParts | part);
     }
     return invalidParts;
 }
 
 IntSize ScrollbarThemeAura::buttonSize(const ScrollbarThemeClient& scrollbar)
 {
     if (scrollbar.orientation() == VerticalScrollbar) {
-        IntSize size = Platform::current()->themeEngine()->getSize(WebThemeEngine::PartScrollbarUpArrow);
-        return IntSize(size.width(), scrollbar.height() < 2 * size.height() ? scrollbar.height() / 2 : size.height());
+        int squareSize = scrollbar.width();
+        return IntSize(squareSize, scrollbar.height() < 2 * squareSize ? scrollbar.height() / 2 : squareSize);
     }
 
     // HorizontalScrollbar
-    IntSize size = Platform::current()->themeEngine()->getSize(WebThemeEngine::PartScrollbarLeftArrow);
-    return IntSize(scrollbar.width() < 2 * size.width() ? scrollbar.width() / 2 : size.width(), size.height());
+    int squareSize = scrollbar.height();
+    return IntSize(scrollbar.width() < 2 * squareSize ? scrollbar.width() / 2 : squareSize, squareSize);
 }
 
 } // namespace blink