Auto merge of #15368 - szeged:get_availability, r=nox

Implement GetAvailability for Bluetooth

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This implements the [getAvailability](https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetooth-getavailability) function from the spec.

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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/webidls/Bluetooth.webidl

@@ -29,8 +29,8 @@ dictionary RequestDeviceOptions {
 
 [Pref="dom.bluetooth.enabled"]
 interface Bluetooth : EventTarget {
-  // [SecureContext]
-  // Promise<boolean> getAvailability();
+  [SecureContext]
+  Promise<boolean> getAvailability();
   [SecureContext]
   attribute EventHandler onavailabilitychanged;
   // [SecureContext, SameObject]