Oilpan: add missing Event-related USED_FROM_MULTIPLE_THREADS() decls.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@168141 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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