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Factor out common code in UpstartClient, and allow UpstartClient to
start arbitrary jobs instead of a fixed set of them.

BUG=b:115779632
TEST=./out/Debug/unit_tests
TEST=./out/Debug/component_unittests
TEST=Ensure dummy job can be started with UpstartClient

Change-Id: I362ad288b93615753cd778e823173d24a9ac16df
Reviewed-on: https://chromium-review.googlesource.com/c/1307238Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Christopher Morin <cmtm@google.com>
Cr-Commit-Position: refs/heads/master@{#604095}