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A new helper called BrowserActivityWatcher is introduced to encapsulate
BrowserList observation. This results in fewer platform macros since
Android doesn't use BrowserList (or NOTIFICATION_BROWSER_[...] before
it).

The structure of ThreadWatcherObserver, particularly startup and
shutdown, was unorthodox so that's also made more idiomatic. The class
can move entirely to the cc file.

Bug: 268984
Change-Id: I46a8948349e62993c96e85963cae3420679ae45f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695831Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678366}