Move the scroll corner logic to RenderLayerScrollableArea

This change moves m_scrollCorner and the associated logic
to the new class. paintScrollCorner is kept as a shim as
it is called from RenderLayerBacking and we don't want to
make it aware of the split yet.

BUG=260899

Review URL: https://chromiumcodereview.appspot.com/23567047

git-svn-id: svn://svn.chromium.org/blink/trunk@158139 bbb929c8-8fbe-4397-9dbb-9b2b20218538