Add a mock member to INTERNAL__HAVE_EMDAWNWEBGPU_HEADER.

This makes the generated `webgpu.h` compliant with the rule saying that a C structure must not be empty.

Bug: chromium:366932309
Change-Id: Idc385a4447204b1c77faf492884f9f6270b87478
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/206634
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/dawn/dawn.json b/src/dawn/dawn.json
index ac8b6c5..d51c44d 100644
--- a/src/dawn/dawn.json
+++ b/src/dawn/dawn.json
@@ -42,7 +42,10 @@
     "INTERNAL__HAVE_EMDAWNWEBGPU_HEADER": {
         "category": "structure",
         "tags": ["emscripten"],
-        "members": []
+        "_comment": "TODO(issues.chromium.org/issues/366932309): The struct must have at least one field.",
+        "members": [
+            {"name": "unused", "type": "bool", "default": "false"}
+        ]
     },
 
     "create instance": {