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This CL ensures that the ModuleDatabase task runner, the inspection
results cache task runner and the module blacklist cache task runner
don't block shutdown.

Also, this CL changes the inspection results cache so that it is updated
atomically.

Bug: 1021904, 1022041
Change-Id: Ia87e13f60e00725583f95c55e231edfd4d7648b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899750Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713191}