Chrome OS: Ash support for >=3 displays.

As the first step, for >=3 displays we always use
horizontal layout and disallow mirroring.

This would allow basic multi-display functionality.

BUG=422161
TEST=trybot

Review URL: https://codereview.chromium.org/657583003

Cr-Commit-Position: refs/heads/master@{#307154}
10 files changed