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This CL just adds an empty mojo interface that will be used for
implementing portals. The interface will be exposed by the browser in a
follow-up CL.

Bug: 865565
Change-Id: I1cd93aa19f83562b5f9dccfe3eb75c93433e6770
Reviewed-on: https://chromium-review.googlesource.com/1144054Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578466}