Oilpan: Remove support for tracing off-heap HashSets.

R=haraken@chromium.org, oilpan-reviews@chromium.org, wibling@chromium.org
BUG=382557

Review URL: https://codereview.chromium.org/402433005

git-svn-id: svn://svn.chromium.org/blink/trunk@178365 bbb929c8-8fbe-4397-9dbb-9b2b20218538