[ExecuTorch][WebGPU] Use requiredFeatures instance API on native + emscripten Dawn

Pull Request resolved: #20652

**Key the `timedWaitAny` instance setup to the actual Dawn API instead of `__EMSCRIPTEN__`, so native-rig Dawn and emscripten/emdawnwebgpu use the modern `requiredFeatures` path and only the vendored Dawn uses the legacy `capabilities.*` path.**

The instance-descriptor setup was guarded by `#if defined(__EMSCRIPTEN__)`, which routed emscripten (emdawnwebgpu, emcc 4.0.19+) through the legacy `capabilities.*` API that no longer exists there. The guard now keys off the API actually present.

Key changes:
- `WebGPUDevice.cpp` — `#if defined(__EMSCRIPTEN__)` → `#if defined(WEBGPU_DAWN_INSTANCE_CAPABILITIES)`. The legacy `instance_desc.capabilities.*` path is taken only by the buck-vendored Dawn (which defines the macro); native cmake Dawn and emscripten leave it undefined and take the `requiredFeatures` / `WGPUInstanceFeatureName_TimedWaitAny` path.

Co-authored-with: Claude Code.
ghstack-source-id: 399812934
@exported-using-ghexport

Differential Revision: [D110149678](https://our.internmc.facebook.com/intern/diff/D110149678/)

Author: JCNTH

backends/webgpu/runtime/WebGPUDevice.cpp

@@ -90,7 +90,9 @@ WebGPUContext create_webgpu_context() {
 
   // TimedWaitAny lets webgpu_wait() block on futures via wgpuInstanceWaitAny.
   WGPUInstanceDescriptor instance_desc = {};
-#if defined(__EMSCRIPTEN__)
+  // Vendored (buck) Dawn uses the older capabilities.* API; the rig's native
+  // Dawn and emscripten's emdawnwebgpu (emcc 4.0.19+) use requiredFeatures.
+#if defined(WEBGPU_DAWN_INSTANCE_CAPABILITIES)
   instance_desc.capabilities.timedWaitAnyEnable = true;
   instance_desc.capabilities.timedWaitAnyMaxCount = 1;
 #else