Replace calls to Document::createEvent() with Event::create calls
Replace calls to Document::createEvent() with Event::create calls

This avoids a funny-looking static_pointer_cast and eliminates the only
caller of createEvent() in core (it should only be called by the bindings).

R=eseidel

Review URL: https://codereview.chromium.org/37683002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@160372 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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