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