[oilpan] Fix OffHeapCollectionTraceTrait

R=ager@chromium.org, haraken@chromium.org, keishi@chromium.org
BUG=

Review URL: https://codereview.chromium.org/157093004

git-svn-id: svn://svn.chromium.org/blink/trunk@166709 bbb929c8-8fbe-4397-9dbb-9b2b20218538