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When the component updater detects that a new Lacros image is available
for download, show a notification in the system tray. This is similar
to what we used to do for Flash component updates.

The only subtlety is that we have to start watching for updates after
the initial install of the lacros component. Otherwise the user gets
an "update available" notification for a binary they just chose to
install by enabling the LacrosSupport flag.

Bug: 1154427
Test: added to ash_unittests and unit_tests
Change-Id: I0cabc8aad7963d07447a23a812ac5ec2f8ffec6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594320
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838723}