Revert of Make ScrollbarThemeAura selectively invalidate scrollbar parts.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
index 0a5e666899de71ff58ee62a7851426356c46ba1f..80820a9a26fe92cb854e65c98f3adf2d76168d04 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
@@ -274,11 +274,5 @@
     return [scrollbarPainter knobAlpha];
 }
 
-ScrollbarPart ScrollbarThemeMacOverlayAPI::invalidateOnThumbPositionChange(const ScrollbarThemeClient& scrollbar, float oldPosition, float newPosition) const
-{
-    // ScrollAnimatorMac will invalidate the thumb when appropriate.
-    return NoPart;
-}
-
 } // namespace blink