Update dawn version to chromium/6594

- This commit updates the version of dawn used in the
webgpu module. Code compiles and runs fine until exit.
- Dawn's cmake has been slightly improved upstream, so
we can discover the monolithic `webgpu_dawn` library
and webgpu headers from the installation directory.
- The `DiscoverDefaultAdapters` and `EnableBackendValidation`
functions are no longer present in the new dawn tag, so this
commit removes their usage.
- There are a few deprecation warnings at runtime and all unit
tests crash during exit during the destruction of static WGPUContext.
- Future commits will address runtime problems.