This changes most callers of ParseScopedStatement to call a new, simpler form
of ParseStatement, which takes only |labels| and |ok|. This allows us to remove
the |legacy| attribute from ParseScopedStatement.

The only remaining caller of ParseScopedStatement is ParseIfStatement.

This patch is a strict refactoring, and should change no behavior.

R=littledan@chromium.org

Review-Url: https://codereview.chromium.org/2699793002
Cr-Commit-Position: refs/heads/master@{#43259}