Auto merge of #27429 - asajeffrey:webxr-user-intent-pref, r=Manishearth

Add a pref to indicate user intent to enter webxr

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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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resources/prefs.json

@@ -50,6 +50,7 @@
   "dom.webxr.layers.enabled": false,
   "dom.webxr.sessionavailable": false,
   "dom.webxr.test": false,
+  "dom.webxr.unsafe-assume-user-intent": false,
   "dom.worklet.timeout_ms": 10,
   "gfx.subpixel-text-antialiasing.enabled": true,
   "gfx.texture-swizzling.enabled": true,