Revert "Make WGPUImageCopy[Buffer|Texture] inextensible"

This reverts commit 11e2e549be4a1a8f19e707f8a459239e0f82173d.

Reason for revert: Roll failures.
There are still usages of nextInChain in
components/optimization_guide/internal/third_party/ml_drift/src/ml_drift/webgpu/webgpu_api_util.cc

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>

TBR=cwallez@chromium.org,kainino@chromium.org,dawn-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: If702312ba618191b8e7630bea7cf854901868bc6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/183429
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
diff --git a/src/dawn/dawn.json b/src/dawn/dawn.json
index a68e7f9..cd838b2 100644
--- a/src/dawn/dawn.json
+++ b/src/dawn/dawn.json
@@ -2311,6 +2311,7 @@
     },
     "image copy buffer": {
         "category": "structure",
+        "extensible": "in",
         "members": [
             {"name": "layout", "type": "texture data layout"},
             {"name": "buffer", "type": "buffer"}
@@ -2318,6 +2319,7 @@
     },
     "image copy texture": {
         "category": "structure",
+        "extensible": "in",
         "members": [
             {"name": "texture", "type": "texture"},
             {"name": "mip level", "type": "uint32_t", "default": "0"},