This brings our implementation closer to the verbage in the spec, which
changed user gesture start to last activation timestamp.
This also adds comments to various parts of WindowContext which interact
with mLastActivationTimestamp nee mUserGestureStart, pointing to the
various parts of the spec which describe this functionality, making it
easier to follow (and giving us SpecMonkey references).
Differential Revision: https://phabricator.services.mozilla.com/D245758
f583e68c6e