Oilpan: Replace RefPtrs to Node and its subclasses in core/events/ with Oilpan transition types.

BUG=357163

Review URL: https://codereview.chromium.org/308073005

git-svn-id: svn://svn.chromium.org/blink/trunk@175126 bbb929c8-8fbe-4397-9dbb-9b2b20218538