[wake-lock] API to get list of active wake locks (#133)

petele has just created a new issue for https://github.com/w3c/wake-lock:

== API to get list of active wake locks ==
It would be helpful to have an function on the `WakeLock` interface that returns the list of active `WakeLockRequest` objects. 

This would prevent uncancellable wake locks if you lose a reference to the wake lock, or it's accidentally garbage collected.

Please view or discuss this issue at https://github.com/w3c/wake-lock/issues/133 using your GitHub account