mus: Changes SetDisplayRoot() to create actual display

It now takes enough information for mus to create the display. Further
we now assume mus has not created the display at the time
SetDisplayRoot() is called.

BUG=708279
TEST=covered by tests
R=kylechar@chromium.org, nasko@chromium.org

Review-Url: https://codereview.chromium.org/2829733002
Cr-Commit-Position: refs/heads/master@{#466057}
23 files changed