Add mutex for GetBuilders and GetBuildSlaves
Add mutex for GetBuilders and GetBuildSlaves

Should have been added in https://codereview.chromium.org/1949183003/

BUG=skia:

Review-Url: https://codereview.chromium.org/1965773002

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