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This updates the installer, and the launchd agent (whenever it is run
as root) to run the upgrade tool.

Bug: 954427
Change-Id: I7f4a4b89c95ec76f706f4906a207af2236c29871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1633970Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664125}