Bug 1614038 part 1. Fix ChannelSplitterNode to follow the spec for its setters. r=padenot

This is based on looking at what the actual spec constraints around the
channelCount, channelCountMode, and channelInterpretation setters are and on
testing Chrome's behavior: the setters _can_ be called as long as you do it with
the current value. That said, the spec sure could use being clearer here; the
style it's using is pretty hard to follow, unfortunately.

Differential Revision: https://phabricator.services.mozilla.com/D62140