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Adds a NewTabWithUserActivation method to open a new tab
with a user activation. This means that the page will
behave as if the user has interacted with it and autoplay
will always be allowed.

This CL also tweaks assistant to use it so that it can
open tabs without the autoplay restriction.

BUG=884777

Change-Id: I7c9327b438ac51003442f0e97bd6e63086fb48cd
Reviewed-on: https://chromium-review.googlesource.com/1239543Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595202}