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Does the following refactoring/cleanup in the XR browser tests:

1. Standardizes the naming of test classes.
2. Switches any easily-ported OpenVR tests to run with both OpenVR and
   WMR runtimes.
3. Deduplicates some code DirectX adapter enumeration code.
4. Ensures that WMR tests always create a test hook before the test
   starts, to avoid issues with the real Mixed Reality Portal starting
   up if it's installed and a test hook is otherwise not needed for a
   test.
5. Documenation clarification.

Bug: 926048
Change-Id: I2af46b3aa103f5b8d80c676154af451e93384a89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1633316
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664467}