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NewBrowserCallback is used from window.open (among other things). As
part of this BrowserFragmentController now has an API to get the list
of BrowserTabControllers and the ability to set a single active one.

BUG=none
TEST=testNewBrowser.testNewBrowser

Change-Id: I55702381c1adee4322a6fc660ce161bcc92c3dca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1897487Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712828}