DevTools: Add simple selector source ranges to CSS.SelectorList (introduce Selector)

This patch also removes CSS.SelectorList.range, as it is now redundant.

R=pfeldman@chromium.org, pfeldman, vsevik
BUG=303080

Review URL: https://codereview.chromium.org/25722003

git-svn-id: svn://svn.chromium.org/blink/trunk@159733 bbb929c8-8fbe-4397-9dbb-9b2b20218538