Remove RenderLayer::scrolledContentOffset

All the callers were moved to RenderBox::scrolledContentOffset.
As RenderBox::scrolledContentOffset rightly ASSERTs that the
renderer has an overclip, several callers were updated to do
so.

BUG=260899

Review URL: https://codereview.chromium.org/27046006

git-svn-id: svn://svn.chromium.org/blink/trunk@159889 bbb929c8-8fbe-4397-9dbb-9b2b20218538