Auto merge of #25030 - szeged:webgpu-constellation, r=gterzian

Only start WebGPU thread if an adapter is requested

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This addresses 1 and 2 from https://github.com/servo/servo/issues/24706#issuecomment-555491438
We send a message to constellation instead of creating the `WebGPU` thread on the start. We send back the result to script and set the `Window`'s `web_gpu` component there.
cc @jdm @imiklos

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components/script/dom/bindings/trace.rs

@@ -524,6 +524,7 @@ unsafe_no_jsmanaged_fields!(WebGLTextureId);
 unsafe_no_jsmanaged_fields!(WebGLVertexArrayId);
 unsafe_no_jsmanaged_fields!(WebGLVersion);
 unsafe_no_jsmanaged_fields!(WebGLSLVersion);
+unsafe_no_jsmanaged_fields!(RefCell<Option<WebGPU>>);
 unsafe_no_jsmanaged_fields!(RefCell<Identities>);
 unsafe_no_jsmanaged_fields!(WebGPU);
 unsafe_no_jsmanaged_fields!(WebGPUAdapter);