Update VertexStepMode::VertexBufferNotUsed jsrepr to undefined

As suggested in an emscripten update,
VertexStepMode::VertexBufferNotUsed will be undefined in jsrepr
in generated files.

Bug: dawn:1000
Change-Id: I3ad96f66f2965d6aa589174225eee538ec97d060
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/97900
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Shrek Shao <shrekshao@google.com>

dawn.json

@@ -1486,7 +1486,7 @@
         "values": [
             {"value": 0, "name": "vertex"},
             {"value": 1, "name": "instance"},
-            {"value": 2, "name": "vertex buffer not used"}
+            {"value": 2, "name": "vertex buffer not used", "jsrepr": "undefined", "valid": true}
         ]
     },
     "load op": {