[VR] Speculative fix for flaky Non-immersive during immersive

testNonImmersiveStopsDuringImmersive is flaky because we receive a non-
immersive rAF callback, while there is an immersive session.
https://crrev.com/c/2354537 ensured that the frame loop will not be
started back until the page has been notified that the session ends, but
it is possible that a non-immersive frame has been scheduled already when
the session returns. This change speculatively patches the test by
allowing at most one frame to be received during an immersive session.
If this removes the flakiness, a more thorough product fix will be
investigated.

Bug: 1116261
Change-Id: I11d8b06c2a9c80503c30a687da97922ef3a04c82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355014


Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798341}