[Refactor] Moved scrollbar creation and deletion to ScrollbarManager

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
index 0eb408c2ca75d28dd60b9d0dd1f261cf87488eae..e604cac194b8006e6ac35c4ac8505ac88869cb46 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
@@ -396,6 +396,8 @@ void ScrollableArea::FinishCurrentScrollAnimations() const {
 
 void ScrollableArea::DidAddScrollbar(Scrollbar& scrollbar,
                                      ScrollbarOrientation orientation) {
+  if (scrollbar.IsCustomScrollbar())
+    return;
   if (orientation == kVerticalScrollbar)
     GetScrollAnimator().DidAddVerticalScrollbar(scrollbar);
   else
@@ -408,6 +410,8 @@ void ScrollableArea::DidAddScrollbar(Scrollbar& scrollbar,
 
 void ScrollableArea::WillRemoveScrollbar(Scrollbar& scrollbar,
                                          ScrollbarOrientation orientation) {
+  if (scrollbar.IsCustomScrollbar())
+    return;
   if (ScrollAnimatorBase* scroll_animator = ExistingScrollAnimator()) {
     if (orientation == kVerticalScrollbar)
       scroll_animator->WillRemoveVerticalScrollbar(scrollbar);