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Biggest non-style/c++11 change is to convert from NotificationRegistrar to
BrowserListObserver.

This should not result in any functional change.

BUG=889097,268984
TEST=covered by tests

Change-Id: I8ea1c89f28c66f312fbcfda925591f96852d2173
Reviewed-on: https://chromium-review.googlesource.com/c/1298123Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602421}