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 955bd6b9728df0ba647394764e0e2f7346bbb794..960d38e1b17e306a5a6c4d987ad600afbdcdff92 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
@@ -104,6 +104,13 @@ static bool supportsExpandedScrollbars()
     return globalSupportsExpandedScrollbars;
 }
 
+ScrollbarPart ScrollbarThemeMacOverlayAPI::invalidateOnThumbPositionChange(
+    const ScrollbarThemeClient& scrollbar, float oldPosition, float newPosition) const
+{
+    // ScrollAnimatorMac will invalidate scrollbar parts if necessary.
+    return NoPart;
+}
+
 void ScrollbarThemeMacOverlayAPI::registerScrollbar(ScrollbarThemeClient& scrollbar)
 {
     ScrollbarThemeMacCommon::registerScrollbar(scrollbar);