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We currently fetch the list of engaged sites on every navigation on the UI
thread. This is slow. We also do this separately for each tab.

This CL introduces a profile keyed service called LookalikeUrlService. This
service fetches the list of engaged sites every 5 minutes in a background thread
and stores the results until the next update. It also gets rid of the need to do
a fetch for each tab separately.

Bug: 913647
Change-Id: I9f7080c45834de576eb081243778f2f17c3e4ccd
Reviewed-on: https://chromium-review.googlesource.com/c/1389167
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622111}