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Because chrome/common/mac/launchd does not wait for process exit, and
because the launchctl process itself may exit prior to setting up the new
service, we must wait at certain points during updater setup to ensure
that services are installed prior to their use.

This CL replaces the naive 3 second wait with a 500ms polling loop that
has a 3 second timeout.

Bug: 1122120
Change-Id: I1c67bba21f223ec0691b55b9fe72eb28a03e98e9
Fixed: 1122120
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429309
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812130}