Add Buffer Destroyed state and validation tests

Adds Destroy() to the Buffer frontend but doesn't implement
it in the backend yet.

Bug: dawn:46, dawn:7
Change-Id: I2e6bdba1484fc5f49599801b4ffe3e9a6aaf60a5
Reviewed-on: https://dawn-review.googlesource.com/c/3720
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
diff --git a/dawn.json b/dawn.json
index b72217f..fa0c664 100644
--- a/dawn.json
+++ b/dawn.json
@@ -185,6 +185,9 @@
             },
             {
                 "name": "unmap"
+            },
+            {
+                "name": "destroy"
             }
         ]
     },