Make WGPUImageCopy[Buffer|Texture] inextensible

Fixed: dawn:2227
Change-Id: I49181b05f3bda2ea1724abadf0ff7513d661c56c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/183240
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/dawn/dawn.json b/src/dawn/dawn.json
index a50d79c..d2cf0de 100644
--- a/src/dawn/dawn.json
+++ b/src/dawn/dawn.json
@@ -2309,7 +2309,6 @@
     },
     "image copy buffer": {
         "category": "structure",
-        "extensible": "in",
         "members": [
             {"name": "layout", "type": "texture data layout"},
             {"name": "buffer", "type": "buffer"}
@@ -2317,7 +2316,6 @@
     },
     "image copy texture": {
         "category": "structure",
-        "extensible": "in",
         "members": [
             {"name": "texture", "type": "texture"},
             {"name": "mip level", "type": "uint32_t", "default": "0"},