[Refactor] Moved scrollbar creation and deletion to ScrollbarManager

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

 /*
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 113 matching lines @@
 
   virtual void DidAddScrollbar(Scrollbar&, ScrollbarOrientation);
   virtual void WillRemoveScrollbar(Scrollbar&, ScrollbarOrientation);
 
   // Called when this ScrollableArea becomes or unbecomes the global root
   // scroller.
   virtual void DidChangeGlobalRootScroller() {}
 
   virtual void ContentsResized();
 
+  virtual LayoutObject* LayoutObjectForScrollbars() const { return nullptr; }

[bokan, 2017/06/27 17:48:52]

Lets call this LayoutObjectForScrollbarStyle. Also, add a TODO to move these out
of ScrollableArea into a new interface class with the 661236 as the crbug #.

+  virtual bool ShouldUseCustomScrollbars(LayoutObject*&) const { return false; }
+
   bool HasOverlayScrollbars() const;
   void SetScrollbarOverlayColorTheme(ScrollbarOverlayColorTheme);
   void RecalculateScrollbarOverlayColorTheme(Color);
   ScrollbarOverlayColorTheme GetScrollbarOverlayColorTheme() const {
     return static_cast<ScrollbarOverlayColorTheme>(
         scrollbar_overlay_color_theme_);
   }
 
   // This getter will create a ScrollAnimatorBase if it doesn't already exist.
   ScrollAnimatorBase& GetScrollAnimator() const;
@@ skipping 318 matching lines @@
   // vertical-lr / ltr            NO                      NO
   // vertical-lr / rtl            NO                      YES
   // vertical-rl / ltr            YES                     NO
   // vertical-rl / rtl            YES                     YES
   IntPoint scroll_origin_;
 };
 
 }  // namespace blink
 
 #endif  // ScrollableArea_h