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This change plumbs through focus (both blink/frame and device) to XRSessions, so we
only call raf callbacks when focused.

BUG=813230

Change-Id: I0aefdce989f6dd60b4cd0acf1a1477fdb2e845b8
Reviewed-on: https://chromium-review.googlesource.com/1011288Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551842}