Add a way for the gpu veto to report back the reason why it said no

BUG=2334
R=bsalomon@google.com

Author: humper@google.com

Review URL: https://codereview.chromium.org/301423002

git-svn-id: http://skia.googlecode.com/svn/trunk/src@15012 2bbb7eff-a529-9590-31e7-b0007b416f81