DevTools: kill WebInspector.SectionCascade class

WI.SectionCascade is another presentation model used by
WI.StyleSidebarPane for rendering its sections.

This patch removes WI.SectionCascade class altogether, moving SSP to
rely on WI.CSSStyleModel.MatchedStyleResult. This is mostly a technical
work.

BUG=537827
R=pfeldman, dgozman

Review URL: https://codereview.chromium.org/1430123003

Cr-Commit-Position: refs/heads/master@{#358745}
6 files changed