Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

cc/input/scrollbar_animation_controller.cc

Index: cc/input/scrollbar_animation_controller.cc
diff --git a/cc/input/scrollbar_animation_controller.cc b/cc/input/scrollbar_animation_controller.cc
index b5a85d703cade5321313c783a2623eb91f183a20..6a8de8c522af87ad11cba09ae0f96655018753cd 100644
--- a/cc/input/scrollbar_animation_controller.cc
+++ b/cc/input/scrollbar_animation_controller.cc
@@ -225,6 +225,10 @@ void ScrollbarAnimationController::WillUpdateScroll() {
     DidScrollUpdate();
 }
 
+void ScrollbarAnimationController::DidRequestShowFromMainThread() {
+  DidScrollUpdate();
+}
+
 void ScrollbarAnimationController::DidResize() {
   StopAnimation();
   Show();