[perf] replace frame count with accurately computed frame rate

[perf] replace frame count with accurately computed frame rate

Compute a frame rate for the period between:

 - User sees first frame of overview animation
 - User sees fully zoomed-out overview

And replace the current Frames count metrics with this. The
previous frame count metrics were actually over the period from
the start of the animation until the window labels finished
animating in; here we are careful to look at a more restricted
period.

https://bugzilla.gnome.org/show_bug.cgi?id=619521