Oilpan: Remove off heap trace trait for HashCountedSet.

First patch towards completely disallowing tracing of off-heap
collections which is needed in order to be able to trace objects
conservatively.

R=wibling@chromium.org
BUG=382557

Review URL: https://codereview.chromium.org/394193002

git-svn-id: svn://svn.chromium.org/blink/trunk@178277 bbb929c8-8fbe-4397-9dbb-9b2b20218538