Re: [deviceorientation] Add API for requesting permission to receive device motion / orientation events (#57)

@anssiko, I am positive on this. Blink's implementation will start as a no-op that immediately resolves with `"granted"` but I agree that it is important for it to be present nonetheless.

@cdumez, can you put together a draft of this change in a PR so we can discuss the specifics on how this will be specified? From the preceding discussion it sounds like we have settled on extending the `DeviceOrientationEvent` and `DeviceMotionEvent` interfaces with something like,

```
enum PermissionState {
  "granted",
  "denied",
};

[Exposed=(Window)]
interface DeviceOrientationEvent {
  static Promise<PermissionState> requestPermission();
};

[Exposed=(Window)]
interface DeviceMotionEvent {
  static Promise<PermissionState> requestPermission();
};
```

These are a specialization of the so-far unspecified `requestPermission()` method that should be added to the Generic Sensor API that will likely have parameters to control things like requested sampling frequency.

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