Currently we are only using a known small set of histograms for the content processes. We should use the format changes now and default to not recording histograms in any child processes. We'll need to add a field for Histograms.json that allows specifying which Histograms should be recorded in which process type.

How many histograms would this change affect?

This would affect most histograms. We still need to clear up whether that is actually ok with the users, sorry for leaving that part out. From what i heard anecdotally from e10s analysis etc., it seems that very few content process histograms are actually being looked at. If that still is the case, we should - as soon as possible - start to default the other histograms to not record there, because reaching out for all individual histograms will be hard.

Looks like we took care of this over in bug 1335343