Convert ARRAYSIZE_UNSAFE -> arraysize in base/.

R=thestig@chromium.org
BUG=423134

Review URL: https://codereview.chromium.org/656033009

Cr-Commit-Position: refs/heads/master@{#299835}
  TestFieldTrialObserver() {
    FieldTrialList::AddObserver(this);
  }

  TestFieldTrialObserver() {
    FieldTrialList::AddObserver(this);
  }
