fix multithread related crashes in skpdiff

BUG=skia:1798
R=mtklein@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/60833002

git-svn-id: http://skia.googlecode.com/svn/trunk@12252 2bbb7eff-a529-9590-31e7-b0007b416f81
12 files changed