Interaction to next paint

The latency of the worst interaction on the page, measured from input to the next frame that reflects it, covering input delay, handler execution and rendering. It replaced first input delay because it measures the whole interaction and every interaction, not just the first and not just the waiting. It is dominated by main-thread work, so the fixes are the ones that make the main thread free: less JavaScript, smaller re-renders, work moved off the critical interaction.