Fix error introduced in r167687 and add copy constructor to PersistentHeapVector.

R=sigbjornf@opera.com, ager@chromium.org, oilpan-reviews@chromium.org

Review URL: https://codereview.chromium.org/181093003

git-svn-id: svn://svn.chromium.org/blink/trunk@167902 bbb929c8-8fbe-4397-9dbb-9b2b20218538