[WebGPU] [Cleanup] Move some function declarations around in WebGPUExt.h
<rdar://problem/104317161>
Pull request: https://github.com/WebKit/WebKit/pull/8711
Committed 258983@main (a70b3d9bdb40): <https://commits.webkit.org/258983@main> Reviewed commits have been landed. Closing PR #8711 and removing active labels.