Reland "Make WGPUImageCopy[Buffer|Texture] inextensible"

This is a reland of commit 11e2e549be4a1a8f19e707f8a459239e0f82173d

Original change's description:
> 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>

Change-Id: I046866aee2ea31d962a2db9e4b5795aef98f1273
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/184400
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/dawn/dawn.json b/src/dawn/dawn.json
index 2a347d1..8234174 100644
--- a/src/dawn/dawn.json
+++ b/src/dawn/dawn.json
@@ -2324,7 +2324,6 @@
     },
     "image copy buffer": {
         "category": "structure",
-        "extensible": "in",
         "members": [
             {"name": "layout", "type": "texture data layout"},
             {"name": "buffer", "type": "buffer"}
@@ -2332,7 +2331,6 @@
     },
     "image copy texture": {
         "category": "structure",
-        "extensible": "in",
         "members": [
             {"name": "texture", "type": "texture"},
             {"name": "mip level", "type": "uint32_t", "default": "0"},