Add webgpu-headers submodule. (#6492)

Fixing the `google` branch after https://github.com/google/iree/runs/3109327734.

third_party/webgpu-headers

diff --git a/third_party/webgpu-headers b/third_party/webgpu-headers
new file mode 160000
index 0000000..c8e0b39
--- /dev/null
+++ b/third_party/webgpu-headers
@@ -0,0 +1 @@
+Subproject commit c8e0b39f6f6f1edded5c4adf7d46aa4d2a95befe