Add Incognito XR browser test

Adds an Incognito version of RequestSessionEntersVr. As a side effect,
adds the ability for tests to override the Browser instance the test
framework uses and a new macro so that it's easy to add Incognito
variations of tests.

Bug: 909942
Change-Id: If58a53185ed6d15bc4439477ea9bec3f6d13962c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1703240
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678034}