[WebGPU] Split Cocoa-specific parts of GPUCanvasContext into GPUCanvasContextCocoa
<rdar://problem/104523434>
Pull request: https://github.com/WebKit/WebKit/pull/8943
Committed 259257@main (5e700122a06a): <https://commits.webkit.org/259257@main> Reviewed commits have been landed. Closing PR #8943 and removing active labels.