Use a RefPtr<HistoryItem> in HistoryFrameLoadSet.

BUG=341220
TEST=http/tests/navigatin/same-and-different-back.html

Review URL: https://codereview.chromium.org/156923002

git-svn-id: svn://svn.chromium.org/blink/trunk@166658 bbb929c8-8fbe-4397-9dbb-9b2b20218538