Fix build due to double declaration of TraceTrait<RefPtr<T> >

Remove the double declaration. See some more discussion here:
  https://codereview.chromium.org/163493003/

NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/163733002

git-svn-id: svn://svn.chromium.org/blink/trunk@167091 bbb929c8-8fbe-4397-9dbb-9b2b20218538