Oilpan: fix build after r177060

Add missing & now-needed V8ValueTraits<> specialization for HeapVector.

R=haraken@chromium.org
BUG=369451

Review URL: https://codereview.chromium.org/350293004

git-svn-id: svn://svn.chromium.org/blink/trunk@177079 bbb929c8-8fbe-4397-9dbb-9b2b20218538