Reland: Make ScrollbarThemeAura selectively invalidate scrollbar parts.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.h

 /*
  * Copyright (C) 2013 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 22 matching lines @@
 
 #include "platform/scroll/ScrollbarThemeMacCommon.h"
 
 typedef id ScrollbarPainter;
 
 namespace blink {
 
 class PLATFORM_EXPORT ScrollbarThemeMacOverlayAPI : public ScrollbarThemeMacCommon {
 public:
     bool shouldRepaintAllPartsOnInvalidation() const override { return false; }
+    ScrollbarPart invalidateOnThumbPositionChange(const ScrollbarThemeClient&, float oldPosiiton, float newPosition) const override;
     void updateEnabledState(const ScrollbarThemeClient&) override;
     int scrollbarThickness(ScrollbarControlSize = RegularScrollbar) override;
     bool usesOverlayScrollbars() const override;
     void updateScrollbarOverlayStyle(const ScrollbarThemeClient&) override;
     WebScrollbarButtonsPlacement buttonsPlacement() const override;
 
     void registerScrollbar(ScrollbarThemeClient&) override;
     void unregisterScrollbar(ScrollbarThemeClient&) override;
 
     void setNewPainterForScrollbar(ScrollbarThemeClient&, ScrollbarPainter);
@@ skipping 11 matching lines @@
 
     bool hasButtons(const ScrollbarThemeClient&) override { return false; }
     bool hasThumb(const ScrollbarThemeClient&) override;
 
     int minimumThumbLength(const ScrollbarThemeClient&) override;
 };
 
 }
 
 #endif