Several cleanups to the output of the debug-only --print-scopes flag:
  - Function name variable only printed once
  - Only print headers for sections of variables that are non-empty
  - Assume Variables stored in Scope::variables_ are never null

R=verwaest@chromium.org

Review-Url: https://codereview.chromium.org/2544063004
Cr-Commit-Position: refs/heads/master@{#41463}