Add Instance and CreateInstance to webgpu.h

This is the first step in making the API before WGPUDevice creation
match webgpu.h and is necessary to implement WGPUSwapChain.

BUG=dawn:269

Change-Id: If92ced42d7683d79e67c02738949ff8b483d22c4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14061
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/dawn.json b/dawn.json
index 30dbbef..3e3fdd9 100644
--- a/dawn.json
+++ b/dawn.json
@@ -738,6 +738,14 @@
             {"value": 1, "name": "uint32"}
         ]
     },
+    "instance": {
+        "category": "object"
+    },
+    "instance descriptor": {
+        "category": "structure",
+        "extensible": true,
+        "members": []
+    },
     "vertex attribute descriptor": {
         "category": "structure",
         "extensible": false,