GitLab

Add the mojo interface to start an OffscreenTab mirroring with the
mirroring service.

Bug: 734672
Change-Id: I3b9edbdccffdfb06b0a325170ed6108be81a6975
Reviewed-on: https://chromium-review.googlesource.com/1195821Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589213}