Disable overlay scrollbars to hide them on non-Mac.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.h b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.h
index 35753a9865532d8ba32a6eb86aee21297e2a0ca4..c12c27fb847d5ef7218dad1a9d37bd617de6cc0a 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.h
@@ -51,6 +51,12 @@ class PLATFORM_EXPORT ScrollbarThemeMac : public ScrollbarTheme {
 
   bool supportsControlTints() const override { return true; }
 
+  // On Mac, the painting code itself animates the opacity so there's no need
+  // to disable in order to make the scrollbars invisible. In fact,
+  // disabling/enabling causes invalidations which can cause endless loops as
+  // Mac queues up scrollbar paint timers.
+  bool shouldDisableInvisibleScrollbars() const override { return false; }
+
   double initialAutoscrollTimerDelay() override;
   double autoscrollTimerDelay() override;