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
Committed:
https://crrev.com/5aeca9582437a4baed70b1a6cf707e654cb3517b
Cr-Commit-Position: refs/heads/master@{#307154}