There are a lot of functions currently NOTREACHED in WebGPUDecoderImpl because they are not actually relevant to the WebGPU decoder implementation.

We should refactor CommonDecoder so that it doesn't have so many GL-related (or otherwise not-WebGPU-relevant) virtual methods in its interface.

It will be easier to do this a little later once we've determined exactly which functions are and aren't needed in the WebGPU decoder.