Enable layout tests paint/ for enable-slimming-paint-v2

Next steps:
- Reduce test result differences if possible;
- Rebaseline tests which produces valid new result;
- Fix bugs.

BUG=644358

Review-Url: https://codereview.chromium.org/2850243002
Cr-Commit-Position: refs/heads/master@{#470163}
1 file changed