Say there's a single lag event, a GC or a busy loop, during which the user
types several characters.
Is this one (lag) event? Several (input) events?
We have INPUT_EVENT_RESPONSE_MS which will accumulate several lagged events in
this case. However, that is more of an indication of how users use Firefox than
how good we've been at eliminating sources of lag.
INPUT_EVENT_RESPONSE_COALESCED_MS records the coalesced time spend waiting for
responses to input events. So in this case it will record one value for the
entire duration of the lag.
MozReview-Commit-ID: H5rYnhwF0q3
28adafda5a