DevTools: [SSP] refactoring to remove styleRule.section property

The patch is a first step towards cleaning up StylesSidebarPane and
introduction of structured object instead of styleRule objects.

This patch removes styleRule.section property, and kills unneeded
section.uniqueProperties list.

R=vsevik, pfeldman

Review URL: https://codereview.chromium.org/699873002

git-svn-id: svn://svn.chromium.org/blink/trunk@184875 bbb929c8-8fbe-4397-9dbb-9b2b20218538