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

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
index 8c9eb392b0874fb227608b3307104731b4e16156..b88ec7fdcd07ca9af7ea02d6f244b0144e643356 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
@@ -326,13 +326,13 @@ ScrollbarPart ScrollbarThemeAura::invalidateOnThumbPositionChange(const Scrollba
 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