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

cdumez has just created a new issue for https://github.com/w3c/deviceorientation:

== Add API for requesting permission to receive device motion / orientation events ==
Due to the privacy issues related to this API (e.g. [https://www.wired.com/story/mobile-websites-can-tap-into-your-phones-sensors-without-asking/](https://www.wired.com/story/mobile-websites-can-tap-into-your-phones-sensors-without-asking/) and [https://arxiv.org/pdf/1605.08763.pdf](https://arxiv.org/pdf/1605.08763.pdf)), we are considering adding to WebKit a permission dialog in order to ask the user if they want to expose their device orientation/motion to the Website.

However, because trackers already register such event listeners on top sites, we are worried about the risk of over-prompting. We wouldn't want this prompt to start showing on a lot of top-sites.

To address the issue, we'd like to propose adding a new API allowing the page's script explicitly ask for permission to access the device / motion. We think we should also require a user-gesture to call this new API.

It could look something like this (similar to Notification API):
```
[Exposed=(Window)]
interface DeviceOrientation {
  [Exposed=Window] static Promise<boolean> requestPermission();
};
```

What are your thoughts?

Please view or discuss this issue at https://github.com/w3c/deviceorientation/issues/57 using your GitHub account