chromeos: move starting UI service for --mus to content

Content effectively depends upon the UI service (aka mus) because
content uses aura, and aura is on top of mus. This moves launching of
the UI service to content (just for the in process case, the --mash
case will be done later). This is a necessary step to have
content_browsertests use the UI service.

BUG=776513
TEST=none

Change-Id: If7ed7b00c12b9fb521fd53c64aa24b398c0f6155
Reviewed-on: https://chromium-review.googlesource.com/754087
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514271}
31 files changed