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If the javascript callback ends the XRSession, then the input events
should stop being processed.  This check is also needed because ending
the session may have changed the state of other variables that are still
being processed, and access violations could occur.

Bug: 967853
Change-Id: Ia54112cdb806e1ed75c32ac9b202dafb85d9b9cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1633749
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664457}