Oilpan: Support for tracing through off-heap supplements.

This CL adds support for tracing through OwnPtr's and adds a trace method to all supplements. Notice that trace is only called on supplements that are properly traced via the Supplementable classes.

R=ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/223763002

git-svn-id: svn://svn.chromium.org/blink/trunk@171060 bbb929c8-8fbe-4397-9dbb-9b2b20218538