[WebGPU] Disable getContext("webgpu") when WebGPU is disabled

https://bugs.webkit.org/show_bug.cgi?id=256533
rdar://109106604

Reviewed by Mike Wyrzykowski, Dan Glastonbury and Tadeu Zagallo.

navigator.gpu is disabled, but getContext("webgpu") isn't.

* Source/WebCore/page/Navigator.cpp:
(WebCore::Navigator::gpu):

Canonical link: https://commits.webkit.org/263877@main

Source/WebCore/page/Navigator.cpp

@@ -366,6 +366,8 @@ GPU* Navigator::gpu()
         auto* frame = this->frame();
         if (!frame)
             return nullptr;
+        if (!frame->settings().webGPU())
+            return nullptr;
         auto* page = frame->page();
         if (!page)
             return nullptr;