Oilpan: move HistoryItem to the heap.

Doing so removes a FrameLoader-reachable Persistent reference to
a DocumentState.

R=
BUG=340522

Review URL: https://codereview.chromium.org/639023004

git-svn-id: svn://svn.chromium.org/blink/trunk@184228 bbb929c8-8fbe-4397-9dbb-9b2b20218538