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Uses the GUARDED_BY annotation for the field we have protected by a
base::Lock in the VR browser test code.

As a result, switches all lock acquiring to AutoLock since the
annotation doesn't currently work with manually acquiring and
releasing.

Bug: 901136
Change-Id: I62b7726ca4ad56778b0354436a8f6d84f47fe86b
Reviewed-on: https://chromium-review.googlesource.com/c/1313752Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604767}