Currently our flutter driver performance tests only have UI thread VSYNC times (frame_build_time_millis in timeline_summary.json), and GPU thread Rasterizer::Draw times (frame_rasterizer_time_millis in timeline_summary.json).

Tracking them separately and ensuring them to be no more than 16ms (for 60fps) only guarantees smoothness, but not latency.

Moreover, something could happen between VSYNC and Rasterizer::Draw.

So as a first step, we could simply track the sum of VSYNC and Rasterizer::Draw times for a frame. In the end, we should directly compute the latency by comparing two timestamps.