Revert r170733 "Oilpan: Replace most of RefPtrs for Event objects with oilpan's transition types "

r170733 broke a lot of tests in oilpan bots.

BUG=
TBR=ager@chromium.org

Review URL: https://codereview.chromium.org/223293003

git-svn-id: svn://svn.chromium.org/blink/trunk@170756 bbb929c8-8fbe-4397-9dbb-9b2b20218538