PerformancePaintTiming: presentationTime property

The presentationTime read-only property of the PerformancePaintTiming interface returns the timestamp when the painted pixels were actually drawn on the screen.

The presentationTime value is nullable: the underlying timestamp is implementation-defined, so browsers that don't populate it return null instead. Because PerformancePaintTiming.paintTime is always populated the same way across implementations, it's the more reliable value to use for metrics that need to be comparable across browsers.