Revert "Separate the scrolling code out of RenderLayer"

The change has caused 2 very bad and widespread hangs
on Canary inside FrameView::removeScrollableArea and
EventHandler::associatedScrollableArea.

Reverting the original change and the crash fix (but leaving
the tests in the tree) so that I can figure out what's wrong
with the change.

TBR=vollick@chromium.org,esprehn@chromium.org
BUG=264588

Review URL: https://codereview.chromium.org/20779002

git-svn-id: svn://svn.chromium.org/blink/trunk@154998 bbb929c8-8fbe-4397-9dbb-9b2b20218538