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

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• (to be filled)

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.

Differential Revision: D110149678

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