In #8714 I attempted to restore webgpu testing to the macOS buildbots by adding automation for building Dawn.

Unfortunately, I discovered that the code would not build against the latest Dawn for several reasons.

1. The API has evolved substantially and required a lot of updating.
2. Even after I got the code to a point where it compiled, an apparent ABI issue halted progress.

We cannot rely on testing webgpu against an old version of Dawn indefinitely. It's not clear that the webgpu backend works on even the current set of browsers.