Reland: Make ScrollbarThemeAura selectively invalidate scrollbar parts.

third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm

 /*
  * Copyright (C) 2010, 2011 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 410 matching lines @@
         currentValue = 1 - progress;
     else
         currentValue = progress;
 
     switch (_featureToAnimate) {
     case ThumbAlpha:
         [_scrollbarPainter.get() setKnobAlpha:currentValue];
         break;
     case TrackAlpha:
         [_scrollbarPainter.get() setTrackAlpha:currentValue];
-        _scrollbar->setTrackNeedsRepaint(true);
+        _scrollbar->setNeedsPaintInvalidation(static_cast<ScrollbarPart>(~ThumbPart));
         break;
     case UIStateTransition:
         [_scrollbarPainter.get() setUiStateTransitionProgress:currentValue];
-        _scrollbar->setThumbNeedsRepaint(true);
-        _scrollbar->setTrackNeedsRepaint(true);
+        _scrollbar->setNeedsPaintInvalidation();
         break;
     case ExpansionTransition:
         [_scrollbarPainter.get() setExpansionTransitionProgress:currentValue];
-        _scrollbar->setThumbNeedsRepaint(true);
+        _scrollbar->setNeedsPaintInvalidation(ThumbPart);
         break;
     }
-
-    _scrollbar->setNeedsPaintInvalidation();
 }
 
 - (void)invalidate
 {
     BEGIN_BLOCK_OBJC_EXCEPTIONS;
     [self stopAnimation];
     END_BLOCK_OBJC_EXCEPTIONS;
     _scrollbar = 0;
 }
 
@@ skipping 741 matching lines @@
         return;
 
     m_visibleScrollerThumbRect = rectInViewCoordinates;
 }
 
 bool ScrollAnimatorMac::canUseCoordinatedScrollbar() {
     return ScrollbarThemeMacCommon::isOverlayAPIAvailable();
 }
 
 } // namespace blink