This measures the wall clock time between a new frame being scheduled in dart code to the Vsync callback in the engine

It's an important source of lag which isn't shown in the top-level UI / Build time graphs, and can correlate with "invisible" missed/non-scheduled frames

I had to change a few unrelated timings in the test, it was only passing based on luck of sort order, and broke when I added more entries to the timeline.

Part of #129150