GitLab

- Reverts the revert in http://crrev/c/1755004, i.e. instantiates the
  collection of persistent histograms.
- Registers a FileMetricsProvider to upload collected metrics.
- Added ios_chrome_metrics_service_client_unittest.mm, patterned after
  chrome_metrics_service_client_unittest.cc.

Bug: 994743
Change-Id: Id8e7ce79f0825eb1cd4c1fb7051709c1d3880cc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752699
Commit-Queue: Peter Lee <pkl@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688154}