This reverts commit 1095eafe.

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.

* Revert "New Flutter Gallery UI (#16936)"

This reverts commit 7038597b.

A new front-end for the Flutter Gallery example.

*  Gallery demo start-time performance test

Current numbers from my Nexus 5:
  "average_frame_build_time_millis": 22.6854,
  "missed_frame_build_budget_count": 15,
  "frame_count": 15

Filed https://github.com/flutter/flutter/issues/3296

@yjbanov @hansmuller @vlidholt