Reorder wgpu::BackendType to match webgpu.h

BUG=dawn:269

Change-Id: I26e2dded1ceb99a34c29924d6f991e30dc6477b6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15280
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/dawn.json b/dawn.json
index 5a6fa82..0038087 100644
--- a/dawn.json
+++ b/dawn.json
@@ -45,13 +45,13 @@
     "backend type": {
         "category": "enum",
         "values": [
-            {"value": 0, "name": "D3D11"},
-            {"value": 1, "name": "D3D12"},
-            {"value": 2, "name": "metal"},
-            {"value": 3, "name": "vulkan"},
-            {"value": 4, "name": "openGL"},
-            {"value": 5, "name": "openGLES"},
-            {"value": 6, "name": "null"}
+            {"value": 0, "name": "null"},
+            {"value": 1, "name": "D3D11"},
+            {"value": 2, "name": "D3D12"},
+            {"value": 3, "name": "metal"},
+            {"value": 4, "name": "vulkan"},
+            {"value": 5, "name": "openGL"},
+            {"value": 6, "name": "openGLES"}
         ]
     },
     "bind group": {