Export slimming paint runtime flag setter

After this, we can set it from the command line through Chromium.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/684653002

git-svn-id: svn://svn.chromium.org/blink/trunk@184485 bbb929c8-8fbe-4397-9dbb-9b2b20218538