The average frame time of page transitions on Moto G4 is now very
close to 16ms (the last 10 measurements on our dashboard are
between 15.5ms to 16.7ms and half of them are below 16ms).
It is now much faster than when we disabled it (which was at about
35ms). So I think that we should be able to enable it by default.
I'll leave the flag there until we implement the retained rendering
to bring the frame time comfortably below 16ms.
See https://github.com/flutter/flutter/issues/13736
When attempting to locate an element in the Flutter Gallery transition
performance driver test, attempt to scroll downward in increments of 25%
of the screen rather than by an absolute pixel amount.
This helps get the test passing on devices with relatively small screen
heights.
