Throw InvalidModificationError if changing pool size after SLD.

[taylor-b]

Fixes #1049.

[aboba]

@taylor-b Since we don't allow connection.peerIdentity, certificates, bundlePolicy or rtcpMuxPolicy to be changed (regardless of whether setLocalDescription has been called yet), it might be simpler just to prohibit changing the iceCandidatePoolSize.

[taylor-b]

@aboba That was my initial thought, but there are some potential situations where changing the size before SLD is desirable. For example, an application may want to create a PeerConnection as early as possible for various reasons, but only set the candidate pool size when a "call" is imminent, since setting it too early would result in STUN/TURN candidates that need to be kept alive. So, this is what the JSEP editors ended up going with.

[aboba]

Makes sense.