webgpu.h: Change VertexStepMode::Undefined to have new behavior #4730

Job	Run time
Go Build and Test	43s
CMake-windows-latest-Debug-msvc	13m 45s
CMake-windows-latest-Release-msvc	16m 16s
CMake-ubuntu-latest-Debug-gcc	20m 28s
CMake-ubuntu-latest-Release-gcc	20m 52s
CMake-macos-latest-Debug-clang	18m 1s
CMake-macos-latest-Release-clang	22m 28s
CMake-macos-13-Debug-clang	45m 39s
CMake-macos-13-Release-clang	53m 0s
	3h 31m 12s

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