Revert "Reland "Make WGPUImageCopy[Buffer|Texture] inextensible""

This reverts commit 4ce34f889e231783348afaedc77988241b2241ab.

Reason for revert: Fix hadn't rolled into Chromium yet >_>

Original change's description:
> 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: I2681191beebed10a694712e250ec631f89bb0438
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/183940
> Auto-Submit: Corentin Wallez <cwallez@chromium.org>
> Reviewed-by: Kai Ninomiya <kainino@chromium.org>
> Commit-Queue: Kai Ninomiya <kainino@chromium.org>

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

Change-Id: I6b7a5b883c0a453986ce9813b83935c0ab180d60
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/184340
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/dawn/dawn.json b/src/dawn/dawn.json
index 47976ab..e891705 100644
--- a/src/dawn/dawn.json
+++ b/src/dawn/dawn.json
@@ -2308,6 +2308,7 @@
     },
     "image copy buffer": {
         "category": "structure",
+        "extensible": "in",
         "members": [
             {"name": "layout", "type": "texture data layout"},
             {"name": "buffer", "type": "buffer"}
@@ -2315,6 +2316,7 @@
     },
     "image copy texture": {
         "category": "structure",
+        "extensible": "in",
         "members": [
             {"name": "texture", "type": "texture"},
             {"name": "mip level", "type": "uint32_t", "default": "0"},