[task driver] Make ReportReceiver thread-safe
[task driver] Make ReportReceiver thread-safe

Change-Id: I3e3fc6ed314e9b40bb6960c0abacbb0567306be5
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/214696
Reviewed-by: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

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