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WebXR may not submit frames if pose data is null.  Pose data will
be null if WebXR isn't visible.  However, we want at least one pose
data to be non-null to know that WebXR is ready to submit frames.

The fix is to hand out a non-null pose data.

Also, fix a crash.  graphics_ is set to null when UI hides, but we
may still have an outstanding frame callback.

Change-Id: Ib304a0b56d8737da1d8200cbb6257645755c99ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540616Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644604}