Add functionality to track metrics about what is being repainted in a frame

This stems from flutter.dev/go/drm-embedder

Question: "Would it be worth exploring a debugger mode that stores metrics about the average area being repainted or prints a history of the area repainted per frame?"

comment from @jacob314: "This would make sense to integrate with Flutter DevTools. Probably both aggregate metrics and metrics for each frame."What was the dirty rect for this frame that was slow?" Why are the dirty rects so large when very little is changing?"

Start by reviewing flutter.dev/go/drm-embedder and the existing DevTools frame-debugging surfaces; the issue names no files or tests. Establish how aggregate and per-frame repaint-area metrics could answer which dirty rect made a frame slow, then define the scope and completion criteria with the DevTools maintainers.

Written by the indexing model from the issue text.