Log save time

Log SaveTiming events, measuring the time it took an edit to be processed
(indicated as time between form submission and the arrival of the first byte of
the response containing the updated page.)

Change-Id: Ia879fa1dccc4ead7239eab916bae03178fe8064f
(cherry picked from commit 4e2cfb51f1848ace2265f41fd6888522e480b75d)