Maybe fix numStencilBits TSAN crash

Bug: skia:7390
Change-Id: Ied9c92147324ddb1245df796cd201df35657bbe0
Reviewed-on: https://skia-review.googlesource.com/143304
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed