Description of handling iceCandidatePoolSize in setConfiguration is out of sync with JSEP

[taylor-b]

In JSEP, we decided that changing the candidate pool size is only allowed before setLocalDescription is called, and as a consequence, the pool will only be used for the first offer/answer exchange.

But webrtc-pc currently has a step in setConfiguration that says simply:

Set the ICE Agent's prefetched ICE candidate pool size as defined in [JSEP] to the value of configuration.iceCandidatePoolSize. If the new ICE candidate pool size changes the existing setting, this may result in immediate gathering of new pooled candidates, or discarding of existing pooled candidates, as defined in [JSEP].

... Which doesn't say what to do if setLocalDescription has already been called.

I'd suggest throwing an InvalidModificationError like we do for everything else.