Change Aura overlay scrollbars from solid color to painted scrollbars.

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 108 matching lines @@
   } else {
     if (part == scrollbar.pressedPart())
       state = WebThemeEngine::StatePressed;
     else if (part == scrollbar.hoveredPart())
       state = WebThemeEngine::StateHover;
   }
 
   return PartPaintingParams(paintPart, state);
 }
 
+static int getScrollbarThickness() {
+  return Platform::current()
+      ->themeEngine()
+      ->getSize(WebThemeEngine::PartScrollbarVerticalThumb)
+      .width;
+}
+
 }  // namespace
 
 ScrollbarTheme& ScrollbarTheme::nativeTheme() {
   if (RuntimeEnabledFeatures::overlayScrollbarsEnabled()) {
-    DEFINE_STATIC_LOCAL(ScrollbarThemeOverlay, theme,
-                        (10, 0, ScrollbarThemeOverlay::AllowHitTest));
+    DEFINE_STATIC_LOCAL(
+        ScrollbarThemeOverlay, theme,
+        (getScrollbarThickness(), 0, ScrollbarThemeOverlay::AllowHitTest));
     return theme;
   }
 
   DEFINE_STATIC_LOCAL(ScrollbarThemeAura, theme, ());
   return theme;
 }
 
 int ScrollbarThemeAura::scrollbarThickness(ScrollbarControlSize controlSize) {
   // Horiz and Vert scrollbars are the same thickness.
   // In unit tests we don't have the mock theme engine (because of layering
@@ skipping 238 matching lines @@
   }
 
   // HorizontalScrollbar
   int squareSize = scrollbar.height();
   return IntSize(
       scrollbar.width() < 2 * squareSize ? scrollbar.width() / 2 : squareSize,
       squareSize);
 }
 
 }  // namespace blink