Bug 958561 - Measure Sync with Firefox Health Report; r=rnewman, a=sylvestre

Metrics for Sync have been added to Firefox Health Report. If Sync is
not configured, we'll report that fact and the supported and preferred
Sync protocols (1.1 or 1.5).

If Sync is configured, we report the daily counts of sync attempts and
how many are successful vs errored. We also report daily counts of the
device types attached to the account.

--HG--
extra : rebase_source : 8bb091e87db99c7ec5b283cba90c21f204577c6a