[WebGPU] Support writing @builtin(frag_depth) from the fragment shader https://gpuweb.github.io/gpuweb/wgsl/#built-in-values-frag_depth
<rdar://problem/105578867>
Pull request: https://github.com/WebKit/WebKit/pull/19634
Committed 269929@main (76a4fde7b09f): <https://commits.webkit.org/269929@main> Reviewed commits have been landed. Closing PR #19634 and removing active labels.