Add attribution information about what triggers paint

[npm1]

From #43, it would be reasonable to add more attribution information to PaintTiming (FP, FCP) similar to the attribution that is exposed in LargestContentfulPaint. Ideally we'll first make sure that there are use cases for this, and then add attribution accordingly.

[noamr]

@npm1: is this in progress? Waiting for feedback?

[npm1]

I think this was filed based on vague feedback. Specifics on use cases and what to expose would be next step.