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Specifically, the test app should install and register itself by:
 1. Call --install on its bundled updater.
 2. Use IPC registration to register the test app's presence.

Also, this CL unifies the test app appid on Win and Mac, and fixes
ownership / initialization issues associated with the test app
initializing services prior to updater installation.

There are also some minor code cleanups around repeated vs once
callbacks.

Change-Id: Ib033be4479ded6b83637f24b133eea4f38004fac
Bug: 1122773
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422138
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809794}