GitLab

Added mojo interface to handle interstitial commands and implemented it
in SupervisedUserNavigationObserver which passes the commands to
SupervisedUserInterstitial when committed interstitials are enabled.

In a future CL an extra class will be added which will listen to JS
commands from the interstitial and passes them via mojo to the
observer.

Bug: 817099
Change-Id: I5285e1fdaa16ee4e96ec7167d470b8ada9b73685
Reviewed-on: https://chromium-review.googlesource.com/950413
Commit-Queue: Carlos IL <carlosil@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542848}