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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