Add a pref to indicate user intent to enter webxr
<!-- Please describe your changes on the following line: -->
Add a pref to indicate that the user has indicated intent to enter webxr.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because it's a command-line pref
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