When one of our end-to-end scroll latency metrics regresses, we need to figure out which part of the pipeline regressed. We can do this for scroll updates, but not for scroll start. We should add ScrollStart versions of all of these metrics.

Event.Latency.TouchToHandled_{Main,Impl}
-> Event.Latency.ScrollStart.TouchToHandled_{Main,Impl}

Event.Latency.ScrollUpdate.HandledToRendererSwap_{Main, Impl}
-> Event.Latency.ScrollStart.HandledToRendererSwap_{Main, Impl}

Event.Latency.ScrollUpdate.RendererSwapToBrowserNotified
-> Event.Latency.ScrollStart.RendererSwapToBrowserNotified

Event.Latency.ScrollUpdate.BrowserNotifiedToBeforeGpuSwap
-> Event.Latency.ScrollStart.BrowserNotifiedToBeforeGpuSwap

Event.Latency.ScrollUpdate.GPUSwap
-> Event.Latency.ScrollStart.GPUSwap