Fix TSAN error for shadow cache

Change-Id: I84b94a19c724e2152d1eac269f425f7f0f2286f5
Reviewed-on: https://skia-review.googlesource.com/17706
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed