Make wgpu::Extent3D default to {1, 1, 1}.

Bug: dawn:22
Change-Id: Ibc9f8acdb5e09453f7b48c409a3f4a32d3927a33
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/30400
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/dawn.json b/dawn.json
index 6f22218..bf8fb54 100644
--- a/dawn.json
+++ b/dawn.json
@@ -719,9 +719,9 @@
     "extent 3D": {
         "category": "structure",
         "members": [
-            {"name": "width", "type": "uint32_t"},
-            {"name": "height", "type": "uint32_t"},
-            {"name": "depth", "type": "uint32_t"}
+            {"name": "width", "type": "uint32_t", "default": 1},
+            {"name": "height", "type": "uint32_t", "default": 1},
+            {"name": "depth", "type": "uint32_t", "default": 1}
         ]
     },
     "fence": {