one more tsan fix

BUG=skia:2460
R=mtklein@google.com
TBR=mtklein

Author: caryclark@google.com

Review URL: https://codereview.chromium.org/255893002

git-svn-id: http://skia.googlecode.com/svn/trunk/src@14382 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed