Add flag for SkGpuSurface creation to enable distance fields.

BUG=skia:2173
R=bsalomon@google.com, reed@google.com, bungeman@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/261783004

git-svn-id: http://skia.googlecode.com/svn/trunk@14525 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed