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.