WebXR Internal Tests

In keeping with the requirements for the wpt_internal tests, these tests validate behavior that is not yet in a stable version of the spec (e.g. AR), test Chrome specific constants, or test things specific to the blink/mojom implementations.

In order to reference xr-internal-device-mocking.js (or other additional files that may be required for an internal test), the additionalChromiumResources variable/array should be set before including webxr_util.js, as webxr_util.js begins loading resources immediately.

To port a test from wpt_internal to external/wpt, the "/webxr/resources/..." paths will need to be converted to "resources/..." paths, and any reliance on internal APIs (i.e. any usage of additionalChromiumResources) should be removed. Those should be the only changes required other than those required by wpt lint.

For more details, please reference the wpt_internal [README] WPT Readme.