Implement QWebEngineHistory serialization

This does uses logic taken from Chromium's state save and restore
code. The history version is incremented from the same method in
QtWebKit in cases where the application would try loading a stream
from a previous version using QtWebKit.

In all cases where Chromium does a restore of a serialized history,
it does so on a fresh WebContents instance, thus we must do the same
and some of the initialization code has to be updated.

Change-Id: I45abb052073bd44c9cb47bc2abcf4b558fe3dbbd
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>