Add --enable-slimming-paint command line flag

Enables the slimming paint work: http://www.chromium.org/blink/slimming-paint
This is dependent on https://codereview.chromium.org/684653002/

Review URL: https://codereview.chromium.org/678763007

Cr-Commit-Position: refs/heads/master@{#301659}
7 files changed