[Not intended to land] Dummy benchmark for platform view scroll and ads banner

[hellohuanlin]

Not intended to land. Just a dummy benchmark while working on platform view performance issues.

Based on https://github.com/lucalooz/flutter_ads_list_perf from #129632, except that I don't use real AdMob (since it's a non-flutter-org repo).

Also I used 320x50 standard banner size, so I can fit 7-8 banners on screen, which should be enough (i guess?).

Feel free to use it when working on platform view performance.
Under devicelab, run

../../bin/dart bin/run.dart --local-engine=ios_profile_unopt --local-engine-host host_profile_unopt -t platform_views_scroll_perf_ios_ad_banners__timeline_summary

[hellohuanlin]

Closing in favor of #145224