Enable Repaint-After-Layout by default.

This CL enables the new repaint-after-layout feature. This changes the way that
we do repainting in Blink and has the potential to cause under-painting.

There are know over-painting issues. The over-painting LayoutTests are being
tracked in crbug.com/340670.

If you see underpainting issues, please try with --disable-repaint-after-layout
and see if the issue persists.

BUG=320139

Review URL: https://codereview.chromium.org/213833019

git-svn-id: svn://svn.chromium.org/blink/trunk@170660 bbb929c8-8fbe-4397-9dbb-9b2b20218538