The JS histograms, too, need to dispatch their accumulations from child to
parent.
JSHistograms_Add now only supports histograms that are in gHistogramsMap or
that were created in the parent process. After bug 1288745, maybe we'll be able
to change this to be less convoluted.
MozReview-Commit-ID: 3qTH89YKbGP
I originally thought we'd be able to avoid the previous implementation's waste
of a map full of every kind of keyed histogram. Unfortunately, other code
(TelemetrySession at the very least) depends on this (and will throw if a keyed
histogram isn't present, even if it is empty)
MozReview-Commit-ID: 8MCGVa595UB
The original commit didn't properly support subsession histograms, so rectify
that lapse by adding support for stripping out the base name of a histogram
when trying to determine its id.
MozReview-Commit-ID: LvUek6f5WUx
Batch the accumulations to only transmit every so often, so we don't incur
too much in the way of IPC overhead penalties.
What this doesn't do:
* remove or restructure child telemetry code to adapt to the new way
* send the telemetry anywhere
* allow for the child process to clear child histograms
* support anything but histograms (but this is expected and okay)
MozReview-Commit-ID: JnUkcmN3Ya7
