Integrate pointer lock with the permissions API

[takumif]

We (on the Chrome team) are working on putting keyboard lock and pointer lock behind permission prompts (Chrome Platform Status).

One piece of feedback that we got from a first-party developer that uses keyboard lock (Chrome Remote Desktop) is that they'd like to have the permission state exposed via the permissions API so that they can update their UI according to the permission state.

We propose to:

1. Modify the section Keyboard.lock() to mention optionally showing a permission prompt and rejecting the promise if the permission is denied.

2. Add a section about integrating with the permissions API, similarly to how other APIs such as window management do.