Fix filediffstore's use of temp dirs
Fix filediffstore's use of temp dirs

BUG=skia:5271

Review-Url: https://codereview.chromium.org/1963493005

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