navigator.getBattery() reports the state of the battery and can be
misused to fingerprint users for a short term. The API was removed from
Firefox, but is still supported in browsers derived from Chromium.
The wrapper mimics Firefox behaviour.
Known bug: Because we mimic Firefox behaviour, a Chromium derived browser
becomes more easily fingerprintable. This can be fixed by properly
BatteryManager.prototype getters and setters.