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Port PrefServiceBridgeTest to PrefServiceTest.
Update docs.
Update OWNERS.

Bug: 1071603
Change-Id: I48f83e1faade5263a6b105268bfd57ded3db27e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307609
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790210}