[Lockdown Mode] Explicitly disable WebGPU

https://bugs.webkit.org/show_bug.cgi?id=267416
<radar://121197420>

Reviewed by Tim Nguyen.

WebGPU is listed as experimental so this behavior exists today, but
specifically disable it when entering lockdown mode to avoid allowing
it enabled unintentionally when the feature is enabled.

* Source/WebKit/WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::adjustSettingsForLockdownMode):

Canonical link: https://commits.webkit.org/278425@main

Source/WebKit/WebProcess/WebPage/WebPage.cpp

@@ -4597,6 +4597,9 @@ void WebPage::adjustSettingsForLockdownMode(Settings& settings, const WebPrefere
     Settings::disableGlobalUnstableFeaturesForModernWebKit();
 
     settings.setWebGLEnabled(false);
+#if HAVE(WEBGPU_IMPLEMENTATION)
+    settings.setWebGPUEnabled(false);
+#endif
 #if ENABLE(GAMEPAD)
     settings.setGamepadsEnabled(false);
 #endif