Bug 875562 - Part 9: Change Health Report to pull from crashes manager; r=bsmedberg, r=rnewman

This patch changes Firefox Health Report to pull crash counts from the
newly-implemented crash manager. The crash manager is now the canonical
source of crash data, so all the code for reading crash dump files from
disk has been removed.

This regresses the collection capability of Firefox Health Report by
removing plugin crashes and hangs from the reported values. This
regression is intended to be temporary and a follow-up bug will be
created to add plugin process event recording to the crash events
system.

This regression has been justified because the old crash reporting
mechanism was severely flawed and wasn't sending accurate data (not all
crashes were being saved to dumps and some dumps would be deleted).

--HG--
extra : rebase_source : 7442a712d06b4d875010e78d4dd4f5cc179c8dfc