Changes to crashes reporting on desktop 30

[Gregory Szorc]

Nightly (and soon to be Aurora 30) have switched how crashes are 
reported [1]. Version 2 of the crashes measurement is documented at [2].

Yes, we stopped recording plugin crashes and hangs and submission 
counts. But the old data wasn't good to begin with. The new data for 
main process crashes should be much more accurate. Plugin events will be 
re-added [3].

Unlike the previous provider, old crashes won't be picked up: only 
crashes added after the crash events code landed (~March 1 for Nightly, 
upgrade to 30 for all other channels) will be reported by FHR.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=875562
[2] 
https://ci.mozilla.org/job/mozilla-central-docs/Tree_Documentation/healthreport/dataformat.html
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=983313