[WebGPU] Disable getContext("webgpu") when WebGPU is disabled
<rdar://problem/109106604>
Pull request: https://github.com/WebKit/WebKit/pull/13644
Committed 263877@main (48a5372923cb): <https://commits.webkit.org/263877@main> Reviewed commits have been landed. Closing PR #13644 and removing active labels.