Fix Dawn WebGPU parallel build race on webgpu_enum_class_bitmasks.h in wasm_inferencing_webgpu_jspi

[Copilot]

Description

Extends cmake/patches/dawn/dawn_parallel_build_fix.patch with a second hunk and updates the explanatory comment in onnxruntime_external_deps.cmake. Two fixes in the patch address the race:

Fix 1 — missing output directory (existing hunk): adds cmake -E make_directory before cmake -E copy in emdawnwebgpu_headers_gen_add so the destination directory is guaranteed to exist before any copy runs, regardless of parallel job scheduling.

Fix 2 — duplicate custom-command recipe (new hunk): removes webgpu_enum_class_bitmasks.h from emdawnwebgpu_cpp's HEADERS list:

 dawn_add_library(
     emdawnwebgpu_cpp
     ...
     HEADERS
         "${EM_BUILD_GEN_DIR}/include/dawn/webgpu_cpp_print.h"
         "${EM_BUILD_GEN_DIR}/include/webgpu/webgpu_cpp.h"
         "${EM_BUILD_GEN_DIR}/include/webgpu/webgpu_cpp_chained_struct.h"
-        "${EM_BUILD_GEN_DIR}/include/webgpu/webgpu_enum_class_bitmasks.h"
     DEPENDS
         emdawnwebgpu_c
 )

emdawnwebgpu_cpp is an INTERFACE library, so anything in HEADERS goes into INTERFACE_SOURCES. When onnxruntime_providers_webgpu links to it, CMake propagates the generated file into ORT's directory scope and emits a second cmake -E copy recipe for the same output path. With -j32, both recipes run concurrently → race → copy failure. Removing the entry eliminates the duplicate recipe. The header remains reachable via ${EM_BUILD_GEN_DIR}/include (already on emdawnwebgpu_c_include's INTERFACE_INCLUDE_DIRECTORIES), and build ordering is preserved through the existing emdawnwebgpu_c → emdawnwebgpu_c_include → emdawnwebgpu_headers_gen chain.

Also removes a stale comment in cmake/onnxruntime_providers_webgpu.cmake that described an older, different bug.

Motivation and Context

The wasm_inferencing_webgpu_jspi step (CI job 88023093249) was failing because webgpu_enum_class_bitmasks.h was being copied by two parallel make jobs simultaneously. The JSPI build is most susceptible because it runs after a warmed ccache, so all ORT compilation steps finish quickly and many more jobs compete for the Dawn header generation step at the same time.

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