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 80820a9a26fe92cb854e65c98f3adf2d76168d04..4eeb99318355c6cbb0f0c16d547a78af7ed92d8f 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMacOverlayAPI.mm
@@ -274,5 +274,11 @@ float ScrollbarThemeMacOverlayAPI::thumbOpacity(const ScrollbarThemeClient& scro
     return [scrollbarPainter knobAlpha];
 }
 
+ScrollbarPart ScrollbarThemeMacOverlayAPI::invalidateOnThumbPositionChange(const ScrollbarThemeClient& scrollbar, float oldPosition, float newPosition) const {
+    // ScrollAnimatorMac will invalidate the thumb when appropriate.
+    // TODO(jbroman): Does this also apply to ScrollbarThemeMacNonOverlayAPI?

[ccameron, 2015/12/23 01:20:37]

In principle it could be "just the thumb", but, in the interest of not breaking
things, just stick with the superclass's "all" behavior.

+    return NoPart;
+}
+
 } // namespace blink