Add a unique frame id and save it on HistoryItem.

This id is currently unused, but is a prerequisite for separating frame tree
state from per-frame state in HistoryItem.

BUG=312929

Review URL: https://codereview.chromium.org/46783003

git-svn-id: svn://svn.chromium.org/blink/trunk@161180 bbb929c8-8fbe-4397-9dbb-9b2b20218538