Get rid of the 'ancestor scroll clipping layer'.

This concept was added to work around a soon-to-be-obsolete limitation of cc:
that you can't be clipped by your scroll parent because it's not your ancestor
in the tree. Unfortunately, this workaround wasn't powerful enough to handle the
general case. You'd need several of this sort of layer to handle nested scroll
parents. This CL kills this unhelpful layer before any poor soul starts
depending on it.

This patch is largely a revert of https://chromiumcodereview.appspot.com/23903012.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23572040

git-svn-id: svn://svn.chromium.org/blink/trunk@158049 bbb929c8-8fbe-4397-9dbb-9b2b20218538