Oilpan: remove invalid asserts from ListHashSet tracing.

When conservatively tracing ListHashSets we can end up tracing the
ListHashSet itself where the ListHashSet backing has been marked
and is on the marking stack for later tracing. Therefore, the
asserts that the backing has already been fully traced if the
backing is marked when we trace the ListHashSet is invalid.

R=erik.corry@gmail.com, oilpan-reviews@chromium.org
BUG=375669

Review URL: https://codereview.chromium.org/336693007

git-svn-id: svn://svn.chromium.org/blink/trunk@176110 bbb929c8-8fbe-4397-9dbb-9b2b20218538