Aura overlay scrollbars shouldn't invalidate thumb on scroll or enabled state.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
index ec03635f57af4bed02a460181197917342f0556c..5c9021bd5f2de31ff0f1cd2627420d1a26bb0bda 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
@@ -92,6 +92,10 @@ class PLATFORM_EXPORT ScrollbarTheme {
     return AllParts;
   }
 
+  // Returns parts of the scrollbar which must be repainted following a change
+  // in enabled state.
+  virtual ScrollbarPart invalidateOnEnabledChange() const { return AllParts; }
+
   virtual void paintScrollCorner(GraphicsContext&,
                                  const DisplayItemClient&,
                                  const IntRect& cornerRect);