Oilpan: add missing Event-related USED_FROM_MULTIPLE_THREADS() decls.

ErrorEvent and ProgressEvent are available in Worker contexts.

R=wibling@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/184413003

git-svn-id: svn://svn.chromium.org/blink/trunk@168141 bbb929c8-8fbe-4397-9dbb-9b2b20218538