Add missing USED_FROM_MULTIPLE_THREADS(IDBVersionChangeEvent) decl.

indexeddb is available in Worker contexts, including this event.

R=
BUG=340522

Review URL: https://codereview.chromium.org/185323003

git-svn-id: svn://svn.chromium.org/blink/trunk@168246 bbb929c8-8fbe-4397-9dbb-9b2b20218538