Lazily generate HistoryItem's serialized form state

Give HistoryItem access to the object that stores the list of form elements
with state, then generated the serialized state when state is requested.

BUG=

Review URL: https://codereview.chromium.org/239993011

git-svn-id: svn://svn.chromium.org/blink/trunk@171961 bbb929c8-8fbe-4397-9dbb-9b2b20218538