[ios] Hide Setting kMetricsReportingWifiOnly behind Feature Flag

UMA over Cellular feature removes kMetricsReportingWifiOnly local pref.
Tests will fail if retrieving or setting occurs.

Bug: 931289
Change-Id: Ibda70e02dde35f50426f6501210130a8c2834be0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574837
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652469}
2 files changed