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This allows us to inject failures to the StartJob and StopJob
functions for better unit test coverage.

BUG=b:142144019
TEST=try

Change-Id: I17ebd3ddd74cd78bb7b123a7fda0a377d53963ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850572Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704473}