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This CL changes the way async callbacks to the lookalike service are
handled. Previously, if the list of engaged sites was out of date, a
background task would be spawned to update them. This could result in
a spike of near-simultaneous background tasks that would contend for the
same lock. This contributes to jank.

This CL changes the process to ensure that only one update is in flight
at a time, and simply pushes the on-updated callbacks onto the end of
the task sequence after any existing update completes.

Bug: 1157596,1135279
Change-Id: I0629f523491d8e4b8e966fa3b65f670c901e7169
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623005
Commit-Queue: Joe DeBlasio <jdeblasio@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842821}