Enable sending OnDisk-backed persistent histograms from previous browser session with stability metrics.

During the rollout of PersistentHistograms we identified some issues with sending up persistent histograms from the previous session. Mainly:
  - Startup performance cost reading the metrics file synchronously
  - Metrics being reported with an incorrect system profile (e.g. histograms only logged for one experiment group being reported with another one)

We should fix both of these and then re-enable the feature.