Add a coarsening function for render timestamps

[noamr]

An enabler for w3c/paint-timing#104.
As per the WebPerfWG discussion, this enables exposing coarsened render times for non-TAO cross-origin images.

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[mmocny]

@noamr Do you still think this should land and then update paint timing to use it?

It looks to me that we just directly wrote the coarsening into Paint timing (here: https://www.w3.org/TR/paint-timing/#mark-paint-timing) and then closed the original issue.

Maybe this is stale? Or could we want to use it in other place?

[noamr]

@noamr Do you still think this should land and then update paint timing to use it?

It looks to me that we just directly wrote the coarsening into Paint timing (here: https://www.w3.org/TR/paint-timing/#mark-paint-timing) and then closed the original issue.

Maybe this is stale? Or could we want to use it in other place?

Yea let's close this