Replace on-stack HashSet<X*> with WillBeHeapHashSet<RawPtrWillBeMember<X>>

BUG=340522

Review URL: https://codereview.chromium.org/379723002

git-svn-id: svn://svn.chromium.org/blink/trunk@177713 bbb929c8-8fbe-4397-9dbb-9b2b20218538