Refactor "track options" to be a dictionary.

This dictionary has members "optional" and "mandatory" for
backwards compatibility; the intent is that constraints using
these members will be treated like old-style constraints, while
constraints without them will be treated like spec-style.

BUG=537590

Review URL: https://codereview.chromium.org/1318393002

Cr-Commit-Position: refs/heads/master@{#353519}
6 files changed