Add a frame sequence number to HistoryItem

This will be used by the history system in content/renderer/ to disambiguate
frames during history navigation.

The current system uses two factors to try to find frames for navigation:
frame routing ids, and frame unique names. Routing ids can't be stored for
later use, because they might get saved to disk and collide with routing ids
in a future renderer process. Unique names are more reliable...unless a
website modifies window.name, at which point we try to do lookups based on a
stale name and can't find the right frame.

Instead, we introduce a frame sequence number. Like the existing item and
document sequence numbers, these can be saved to disk with a low chance of
collisions. These can then be reliably used during the current session to
match frames for back/forward navigations.

BUG=393227,395097

Review URL: https://codereview.chromium.org/418103003

git-svn-id: svn://svn.chromium.org/blink/trunk@178947 bbb929c8-8fbe-4397-9dbb-9b2b20218538