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This change removes a synchronous call from browser code into VRDevice,
in anticipation of making VRDevice a mojo interface.

Browser-side now controls whether a device is fallback.

The change here is temporary until further refactors to support XR.
Fallback devices will be removed, and requestSession will have more logic
to determine which device should be returned.

Change-Id: I4707eecf3a0ca8356f7139934c2e1fc79f934d37
Reviewed-on: https://chromium-review.googlesource.com/1080039Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563057}