Add the ability to watch for a tab that hasn't opened yet.

This CL extends InteractionSequenceBrowserUtil to be able to watch for
new pages and browsers that are opened, and assign TrackedElements to
those (which can then be used to drive an InteractionSequence).

Also adds a number of appropriate unit and regression tests.

Change-Id: I14116e8c03ed3b4ac4db1ef7fea2ca3924c6dd10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3536683
Reviewed-by: David Pennington <dpenning@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#986250}