Mark `depth slice` as Dawn only.

The `depthSlice` argument hasn't landed into the spec yet and is still
up for discussion. Mark as dawn only for now.

Change-Id: Iae4b442e2ba2f6132028136f4705d18bd6140302
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/148160
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
diff --git a/dawn.json b/dawn.json
index 3371066..afa734e 100644
--- a/dawn.json
+++ b/dawn.json
@@ -2411,7 +2411,7 @@
         "extensible": "in",
         "members": [
             {"name": "view", "type": "texture view", "optional": true},
-            {"name": "depth slice", "type": "uint32_t", "default": "0"},
+            {"name": "depth slice", "type": "uint32_t", "default": "0", "tags": ["dawn"]},
             {"name": "resolve target", "type": "texture view", "optional": true},
             {"name": "load op", "type": "load op"},
             {"name": "store op", "type": "store op"},