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}
  static base::FilePath install_dir() {
    base::FilePath result;
    PathService::Get(DIR_SW_REPORTER, &result);
    return result;
  }

  static base::FilePath install_dir() {
    base::FilePath result;
    PathService::Get(DIR_SW_REPORTER, &result);
    return result;
  }
