Add functionality to calculate average Build time and Raster time per frame

While trying to do some performance optimizations on the Wonderous app (https://github.com/gskinnerTeam/flutter-wonderous-app/pull/31), I found myself wanting to select a group of frames from the flutter frames chart on the performance page, and see stats about the average Build and Raster time for that set of frames. You can imagine a shift / cmd click workflow where you can highlight multiple frames and exclude the outliers to get some aggregated stats.

Start with the performance page's Flutter frames chart and trace how frames are selected and displayed. Define the selection interaction described in the issue, then verify that selected frames can exclude outliers and show average Build and Raster times per frame. The payload names no files or tests.

Scritto dal modello di indicizzazione a partire dal testo della issue.