Add a flag for Windows Mixed Reality support

Bug: 924324
Change-Id: Iba37d03e0a6e8d8f20115d0a22fbf0318227af68
Reviewed-on: https://chromium-review.googlesource.com/c/1428096
Commit-Queue: Bill Orr <billorr@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625048}
7 files changed