* cleanup macrobenchmark duplicate files

* add docs about writing/running web benchmarks

* explain how to run like devicelab

* add WidgetBuildRecorder docs

For https://github.com/flutter/flutter/issues/24712

This test verifies that https://github.com/flutter/engine/pull/6923 will speedup the average rasterize time of this test from ~150ms to ~10ms

Please see non-auto-generated files in https://github.com/flutter/flutter/pull/25381/commits/37b21d9fb4756a8d8d12ae0def1d0640740c8818