At a quick glance, one could easily think of the "engine_time" as the
GPU thread time and the "frame_time" as the UI thread time because the
GPU thread time is mainly spent on the engine while the UI thread time
is mainly spent on the Dart framework to generate the frame.
But it's actually the other way. The "engine_time" is UI thread time and
the "frame_time" is the GPU thread time.
To avoid the confusion, rename them to "ui_time" and "raster_time"
respectively. I avoided the "gpu_time" because the rasterization may be
purely on a CPU backed software Skia backend.
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