Oilpan: have RepeatEvent::trace() delegate to Event::trace()

For consistency, have RepeatEvent 'upcall' to Event when tracing.

R=
BUG=340522

Review URL: https://codereview.chromium.org/219283007

git-svn-id: svn://svn.chromium.org/blink/trunk@170516 bbb929c8-8fbe-4397-9dbb-9b2b20218538