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

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Convert remaining native tests to GTest #20706
• -> [ExecuTorch][WebGPU] Use requiredFeatures instance API on native + emscripten Dawn #20652
• [ExecuTorch][WebGPU] 2D-fold mul + permute dispatch (lift 65535 1D cap) #20651
• [ExecuTorch][WebGPU] 2D compute dispatch tests — prefill golden + fold unit test #20584
• [ExecuTorch][WebGPU] 2D compute dispatch — lift the 65535 per-dim cap (prefill path) #20583
• [ExecuTorch][WebGPU] Dynamic-shape integration test (allocate-at-max + per-op resize) #20582
• [ExecuTorch][WebGPU] Dynamic resize hook for slice_copy (dynamic start/end) #20581
• [ExecuTorch][WebGPU] Dynamic resize hook for SDPA (live seq-len S) #20580
• [ExecuTorch][WebGPU] Dynamic resize hook for view_copy #20579
• [ExecuTorch][WebGPU] Dynamic resize hooks for sigmoid and select_copy #20578
• [ExecuTorch][WebGPU] Dynamic resize hooks for add and mul #20577
• [ExecuTorch][WebGPU] Dynamic resize hook for linear_q4gsw #20576
• [ExecuTorch][WebGPU] Dynamic resize hooks for rms_norm, embedding, rope #20575
• [ExecuTorch][WebGPU] Dynamic tensor-shape resize engine core #20574
• [ExecuTorch][WebGPU] SymInt arithmetic ops (add/sub/mul/floordiv) for dynamic shapes #20573

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.
@exported-using-ghexport

Differential Revision: D110149678

Differential Revision: D110149678

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