GitLab

The root windows should use the FrameSinkId of the compositor. In regular
aura, compositors get valid FrameSinkId from context_factory_private().
If window server is hosting viz, then use the id from that.

BUG=754872

Change-Id: I7b1de89f9575ebd532042ffcd0b3a8058d85f251
Reviewed-on: https://chromium-review.googlesource.com/847333
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526735}