Remove old invalidation code path.

This CL removes the old invalidation code path in favour of making the
RepaintAfterLayout path the only path.

This does not remove the entry in RuntimeEnabledFeatures. I'll remove the
code in chrome for --enable-repaint-after-layout and
--disable-repaint-after-layout first and put up a second CL to remove the
entry from RuntimeEnabledFeatures.

BUG=385281

Review URL: https://codereview.chromium.org/336943004

git-svn-id: svn://svn.chromium.org/blink/trunk@176875 bbb929c8-8fbe-4397-9dbb-9b2b20218538