Fix another race in status' BuildCache
Fix another race in status' BuildCache

BUG=skia:

Review-Url: https://codereview.chromium.org/2159193002

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