DevTools: decouple model logic from presentation within CSSTrackerView

This simplifies the rules usage and stylesheet processing logic in
CSSTrackerView from the presentation code that lays a way for future
splitting this view into several parts.
This also groups rules by styleshetts early that simplifies processing
in several places.

Review-Url: https://codereview.chromium.org/2705433003
Cr-Commit-Position: refs/heads/master@{#451429}
4 files changed