Add default to VertexState.bufferCount

Bug: dawn:764
Change-Id: Ib4fe94b198c6980c8dd849b3bb15f88e7f063c5f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/47980
Auto-Submit: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/dawn.json b/dawn.json
index 5b7d2c9..bea338b 100644
--- a/dawn.json
+++ b/dawn.json
@@ -1639,7 +1639,7 @@
         "members": [
             {"name": "module", "type": "shader module"},
             {"name": "entry point", "type": "char", "annotation": "const*", "length": "strlen"},
-            {"name": "buffer count", "type": "uint32_t"},
+            {"name": "buffer count", "type": "uint32_t", "default": 0},
             {"name": "buffers", "type": "vertex buffer layout", "annotation": "const*", "length": "buffer count"}
         ]
     },