Connect mojom::DisplayController from ash to ui.

Connect mojom::DisplayController and the debug keyboard accelerator to
add or remove a display. The accelerator is triggered in mojo:ash and
the display change happens in mojo:ui. Remove the IPC response as it's
unneeded.

The actual adding/removing of a display isn't fully working yet but the
accelerator and Mojo IPC works as intended.

BUG=611475

Review-Url: https://codereview.chromium.org/2323393003
Cr-Commit-Position: refs/heads/master@{#418254}
12 files changed