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The device_utils.ListProcesses method already does most of the work
required to implement GetPidsToTrack. Use that rather than hand-rolling
yet another parse-grepping through the output of `ps`.

Bug: catapult:#3215
Bug: catapult:#4103
Change-Id: I3825761ce2efc6cbdd940f41505789b61506bd8b
Reviewed-on: https://chromium-review.googlesource.com/955628Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542112}