Fix race condition in filediffstore_test. Exposed with Go 1.5.
Fix race condition in filediffstore_test. Exposed with Go 1.5.

BUG=skia:

Review URL: https://codereview.chromium.org/1413523009

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot