Change wgpu::VertexStepMode::VertexBufferNotUsed enum value

When wgpu::VertexStepMode::VertexBufferNotUsed enum that must
be set to empty vertex buffer slots was introduced its value
was temporarily set to zero to pass some CTS tests by making
the use of zero-initialization without any change in Blink.

We applied a patch to Blink to explicitly set
wgpu::VertexStepMode::VertexBufferNotUsed to empty vertex
buffer slots so we can set the enum value to non-zero.

("Undefined") Zero enum value should be used as the
default as specified in JS" so the APIs match more closely.
The default vertex step mode in VertexBufferLayout is
"vertex" then wgpu::VertexStepMode::VertexBufferNotUsed enum
value shouldn't be zero.

Bug: dawn:1000
Change-Id: Iffbc96d4c4e032673faa37894998e4e9944c3ba8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/91920
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Takahiro <hogehoge@gachapin.jp>
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

dawn.json

@@ -1481,9 +1481,9 @@
     "vertex step mode": {
         "category": "enum",
         "values": [
-            {"value": 0, "name": "vertex buffer not used"},
+            {"value": 0, "name": "vertex"},
-            {"value": 1, "name": "vertex"},
+            {"value": 1, "name": "instance"},
-            {"value": 2, "name": "instance"}
+            {"value": 2, "name": "vertex buffer not used"}
         ]
     },
     "load op": {