Fix TestListener in Source/web/tests/CustomEventTest.cpp

It was getting converted to a V8AbstractEventListener in V8GCController, and
that's not good, since it didn't inherit it.

BUG=317669

Review URL: https://codereview.chromium.org/68483003

git-svn-id: svn://svn.chromium.org/blink/trunk@161805 bbb929c8-8fbe-4397-9dbb-9b2b20218538