Correct check for WebGPU support

Just testing for the presence of navigator.gpu is not sufficient to
establish WebGPU support: in particular, at time of writing, Chrome on
Android exposes a navigator.gpu but does not return anything from
requestAdapter.

Context: microsoft#15796 (comment)

js/web/lib/wasm/jsep/init.ts

@@ -132,7 +132,8 @@ class ComputeContextImpl implements ComputeContext {
 
 export const init = async(module: OrtWasmModule, env: Env): Promise<void> => {
   const init = module.jsepInit;
-  if (init && navigator.gpu) {
+  const hasGpu = !!(navigator.gpu && await navigator.gpu.requestAdapter())
+  if (init && hasGpu) {
     if (!env.wasm.simd) {
       throw new Error(
           'Not supported for WebGPU=ON and SIMD=OFF. Please set `env.wasm.simd` to true when using WebGPU EP');