Switch ResizerAreaSet to use RenderBoxes

As part of moving the resizer logic to RenderLayerScrollableArea,
we need to move ResizerAreaSet away from holding RenderLayer. As
we don't want to tie non-rendering to internal details, it was
moved to RenderBox.

One step-back from this change is that we have to check that
the renderer is a RenderBox in RenderLayer. Once the switch to
RenderLayerScrollableArea, this check will be removed for good.

BUG=260899

Review URL: https://chromiumcodereview.appspot.com/24659002

git-svn-id: svn://svn.chromium.org/blink/trunk@158416 bbb929c8-8fbe-4397-9dbb-9b2b20218538