[WebGPU] Fixup WEBGPU_IMPLEMENTATION macros
<rdar://problem/104524084>
Pull request: https://github.com/WebKit/WebKit/pull/8945
Pull request: https://github.com/WebKit/WebKit/pull/9059
Committed 259305@main (7f4a9441ef2f): <https://commits.webkit.org/259305@main> Reviewed commits have been landed. Closing PR #8945 and removing active labels.