Update data reduction proxy statistics prefs less often on desktop

Update data reduction proxy statistics prefs less often on desktop

For platforms other than iOS and Android, the preference data is stored in
memory and written to disk at 60
minute intervals and on shutdown via the DataReductionProxyDelayedPrefService.

For iOS and Android, the DataReductionProxyDelayedPrefService takes a delay time
of zero and simply passes the prefs through to the PrefService.

Review URL: https://codereview.chromium.org/473723002

Cr-Commit-Position: refs/heads/master@{#295521}