[Refactor] Moved scrollbar creation and deletion to ScrollbarManager

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

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
index 1878aed0040caf1982cd3ed901de239c95af20b6..1d458febfe0c82072c802eef1227f7f9a823db2c 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
@@ -131,6 +131,9 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin,
 
   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);