Remove usage of ws::Display in ws::UserDisplayManager.

For external mode we do not have one ws::Display per display::Display.
Have ws::UserDisplayManager take in a display::Display directly instead
of getting them from ws::Display. Also get list of current displays from
display::Screen instead of ws::DisplayManager.

BUG=693081

Review-Url: https://codereview.chromium.org/2692863009
Cr-Commit-Position: refs/heads/master@{#451841}
12 files changed