Scrollbar no longer inherits from FrameViewBase.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index a8cd0c368cf6e3f4833c6d9f6450754fac81772c..23d1185d2bfdf45e79abc1c9582fa4eae0e38004 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1991,7 +1991,7 @@ Scrollbar* PaintLayerScrollableArea::ScrollbarManager::CreateScrollbar(
         ScrollableArea(), orientation, scrollbar_size,
         &ScrollableArea()->Box().GetFrame()->GetPage()->GetChromeClient());
   }
-  ScrollableArea()->Box().GetDocument().View()->AddChild(scrollbar);
+  ScrollableArea()->Box().GetDocument().View()->AddScrollbar(scrollbar);
   return scrollbar;
 }
 
@@ -2013,7 +2013,7 @@ void PaintLayerScrollableArea::ScrollbarManager::DestroyScrollbar(
   if (!scrollbar->IsCustomScrollbar())
     ScrollableArea()->WillRemoveScrollbar(*scrollbar, orientation);
 
-  ToFrameView(scrollbar->Parent())->RemoveChild(scrollbar.Get());
+  ScrollableArea()->Box().GetDocument().View()->RemoveScrollbar(scrollbar);

[szager1, 2017/04/21 08:37:52]

Can't this still be accessed via scrollbar->Parent() ?  That would be simpler
and easier to understand.

[joelhockey, 2017/04/21 09:56:10]

No.  The visibility between core and platform is at the heart of what makes this
change quite tricky.

scrollbar->Parent is of type FrameViewBase.  The RemoveScrollbar method belongs
to FrameView.  I want to get away from the downcasting.

[szager1, 2017/04/21 10:22:36]

I think the proper fix is to define Scrollbar ownership semantics in
FrameViewBase rather than FrameView.  Ideally, this would look something like:

- Move ScrollbarManager from paint/ to platform/
- Make sure ScrollbarManger has all methods that Scrollbar needs
- Make Scrollbar::parent_ a ScrollbarManager

With that in place, all you need to do is move all remaining methods from
FrameViewBase to FrameView, and declare victory.

[szager1, 2017/04/21 10:42:30]

Just to be totally clear, this line of code would become:

scrollbar->Manager()->RemoveScrollbar(scrollbar)

... or even:

scrollbar->RemoveFromManager()

[joelhockey, 2017/04/21 12:22:05]

Yes, nice idea.  I'll have a go at that.  I still need to get
https://codereview.chromium.org/2832883003 landed first when you have some time
to look at it.

   scrollbar->DisconnectFromScrollableArea();
   scrollbar = nullptr;
 }