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The XRFrameDataProvider now returns an associated
XREnvironmentDataProvider interface so that the
two share callback queues and thus allow strict
ordering of the two interfaces. This is critical
for frame synchronization between frames and the
associated environment data.

Note: We can't just mark the interfaceptrs for
XRFrameDataProvider and
XREnvironmentIntegrationProvider as associated
in the XRSession struct. XRDevice implementations
mostly live on separate threads from the
XRFrameDataProviders, so we'd have extra thread
hopping. For the VR headsets we explicitly live
off the main thread to avoid latency, and because
we do some work that may block the thread the
XRFrameDataProvider lives on (for example waiting
for vsync, or submitting frames to headset APIs).

Bug: 867057, 876135, 843376
Change-Id: If2fb62fcd185825209dec08e421df05f34d41c30
Reviewed-on: https://chromium-review.googlesource.com/c/1171794
Commit-Queue: Max Rebuschatis <lincolnfrog@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605545}