WebGPU: Fix `navigator` usage. (#29965)

examples/jsm/capabilities/WebGPU.js

@@ -1,5 +1,4 @@
-let isAvailable = navigator.gpu !== undefined;
-
+let isAvailable = ( typeof navigator !== 'undefined'  && navigator.gpu !== undefined );
 
 if ( typeof window !== 'undefined' && isAvailable ) {