[oilpan] Fix NeedsTracing trait for heap collections nested in other collections

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

Review URL: https://codereview.chromium.org/170433002

git-svn-id: svn://svn.chromium.org/blink/trunk@167339 bbb929c8-8fbe-4397-9dbb-9b2b20218538