Use a new field trial group to enable the SWReporter

BUG=533484

Review URL: https://codereview.chromium.org/1376803002

Cr-Commit-Position: refs/heads/master@{#351788}
 bool SRTPromptNeedsElevationIcon() {
  return !base::StartsWith(
      base::FieldTrialList::FindFullName(kSRTElevationTrial),
      kSRTElevationAsNeededGroup, base::CompareCase::SENSITIVE);
}

bool IsSwReporterEnabled() {
  return !base::StartsWith(
      base::FieldTrialList::FindFullName(kSRTReporterTrial),
      kSRTReporterOffGroup, base::CompareCase::SENSITIVE);
 }

 bool SRTPromptNeedsElevationIcon() {
  return base::FieldTrialList::FindFullName(kSRTElevationTrial) !=
         kSRTElevationAsNeededGroup;
 }
