Make synthetic PerformanceMeasures from each PerformanceMark, to get it logged with an absolute start time and a duration. This makes it fit with the mechanism for generating statistics used by normal timing measurements.
Add null checks for mark list and measure list returned by browser.

FF, unlike Chrome, seems to return null, not an empty array, when querying for user measurements on a page where non are present.
Add entry about user timings.