See the trace with startup-category-enabled here: https://github.com/catapult-project/catapult/files/1725543/pageload-breakdown-traces.tar.gz

There is a top level content::Start slice in CrRendererMain (also in CrGpuMain.) Since this slice executes outside of the message loop and is never closed, it becomes the supreme top level task on the main thread, and all other tasks are descendants of it. Consequently in our metrics, time that should have been categorized as 'idle' gets erroneously categorized as 'startup'.