change all_rAF_time.html timestamps

[becky-gilbert]

This is an experiment to see if changing the timestamps will improve display duration accuracy in the rAF time version.
We were using the timestamp ts that is automatically passed to rAF callbacks, but this timestamp is the time that the frame's main thread starts. Instead, I'm wondering if it would help to get a performance.now() timestamp inside the callback function. See here under 'Addendum about the rAF timestamp', and in the 'Life of a frame' figures, I think this is the difference between 'Begin frame' and 'rAF'.
I think the timing of the frame's main thread might be more consistent over time/frames, so I'm still using the automatically-passed ts timestamps to determine the frame durations.
If this looks an improvement then I can change the time + frames version as well.