This change adds PageState.extra_data, which is an arbitrary

string-to-Any dictionary that callers can use to store whatever data

they need. This works well as an alternative to the pattern of trying to

attach data as an attribute to an HttpRequest, helping keep it nicely

contained and easier to work with.
PageState does absolutely nothing with this field other than

initializing it to an empty dictionary.

Unit tests pass.
Made use of this in some in-progress code in Review Board.