ensure that we call onUnlock only when we onLock succeeded

BUG=
R=halcanary@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/112783004

git-svn-id: http://skia.googlecode.com/svn/trunk@12642 2bbb7eff-a529-9590-31e7-b0007b416f81
6 files changed