Remove the private methods from ScrollableArea on RenderLayer

These methods were not used as they were private so could be
safely removed. While cleaning up, removed also some extra
unused scaffolding methods from the refactoring and moved
m_cachedOverlayScrollbarOffset to RenderLayerScrollableArea.

BUG=260899

Review URL: https://codereview.chromium.org/25577002

git-svn-id: svn://svn.chromium.org/blink/trunk@158667 bbb929c8-8fbe-4397-9dbb-9b2b20218538