[WebGPU] Move getPreferredFormat() from GPUCanvasContext to GPU to align with spec
Pull request: https://github.com/WebKit/WebKit/pull/2337
getPreferredFormat -> getPreferredCanvasFormat https://www.w3.org/TR/webgpu/#dom-gpu-getpreferredcanvasformat
<rdar://problem/97279706>
Committed 252658@main (1e3ea2a7462c): <https://commits.webkit.org/252658@main> Reviewed commits have been landed. Closing PR #2337 and removing active labels.