Improve the HeapListHashSet no-weakness static assert message.

Mention the alternate HeapLinkedHashSet<> if WeakMember<>s are
attempted used with HeapListHashSet<>; for non-trivial reasons,
we only support weakness with the former.

R=
BUG=614112

Review-Url: https://codereview.chromium.org/2034423002
Cr-Commit-Position: refs/heads/master@{#397999}
1 file changed