Add a SlimmingPaint runtime flag and skeleton prototype

Building on the patch in issue 604893003.

This is prototype code and will change significantly in the near future.

Review URL: https://codereview.chromium.org/638143004

git-svn-id: svn://svn.chromium.org/blink/trunk@183506 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Unprefix usage of -webkit-transform throughout Blink

BUG=421268

Review URL: https://codereview.chromium.org/631763005

git-svn-id: svn://svn.chromium.org/blink/trunk@183507 bbb929c8-8fbe-4397-9dbb-9b2b20218538