PerformanceContainerTiming: presentationTime property

The presentationTime read-only property of the PerformanceContainerTiming interface returns the timestamp when the container's newly painted content was 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 PerformanceContainerTiming.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.