Move variations prefs into the variations component

These prefs are used by variations code that is targeted for componentization.

BUG=382865
TBR=thakis

Review URL: https://codereview.chromium.org/1265423003

Cr-Commit-Position: refs/heads/master@{#343661}
  void EnableMetricsReporting() {
    is_metrics_reporting_enabled_ = true;
  }

  void EnableMetricsReporting() {
    is_metrics_reporting_enabled_ = true;
  }
