[dawn][headers] Makes VertexBUfferLayout and VertexAttribute extensible.

Bug: 378866373
Change-Id: Ib253bb49ce6ddae361ac0d975524c8a1dca5b06a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/224494
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/dawn/dawn.json b/src/dawn/dawn.json
index 30bb8ce..99561d5 100644
--- a/src/dawn/dawn.json
+++ b/src/dawn/dawn.json
@@ -2456,7 +2456,7 @@
     },
     "vertex attribute": {
         "category": "structure",
-        "extensible": false,
+        "extensible": "in",
         "members": [
             {"name": "format", "type": "vertex format"},
             {"name": "offset", "type": "uint64_t"},
@@ -2465,7 +2465,7 @@
     },
     "vertex buffer layout": {
         "category": "structure",
-        "extensible": false,
+        "extensible": "in",
         "members": [
             {"name": "array stride", "type": "uint64_t"},
             {"name": "step mode", "type": "vertex step mode"},