Oilpan: Move AnimationNode::EventDelegate to oilpan's heap

This CL removes raw Element* pointers from the EventDelegate hierarchy.

BUG=340522

Review URL: https://codereview.chromium.org/427673004

git-svn-id: svn://svn.chromium.org/blink/trunk@179119 bbb929c8-8fbe-4397-9dbb-9b2b20218538