Add a new string-valued, Rust-visible pref, `dom.webgpu.wgpu-backend`, to `StaticPrefList.yaml` in `modules/libpref/init`. If this string is non-empty, use it to enable the indicated WebGPU backends when we create a `wgpu_core::Global` in `gfx/wgpu_bindings/src/server.rs`. See `StaticPrefList.yaml` for details.
As this is the first time we have exposed a `mirror: always`, `type: DataMutexString` pref to Rust, make the appropriate changes to `generate_static_pref_list.py` to generate the necessary C++ and Rust code to lock the value, make a string copy, and pass ownership back to Rust as an `nsstring::nsACString` (from the `xpcom/rust/nsstring` crate).
Differential Revision: https://phabricator.services.mozilla.com/D173335
46ff1ae846