Oilpan: Change persistent handles in the Event hierarchy to members

Persistent handles in on-heap objects are not allowed (because it's likely to cause memory leaks). This CL removes them from the Event hierarchy, which was moved to oilpan's heap in r169941.

BUG=340522

Review URL: https://codereview.chromium.org/208743011

git-svn-id: svn://svn.chromium.org/blink/trunk@170022 bbb929c8-8fbe-4397-9dbb-9b2b20218538