DevTools: Decouple CSS model from UI entities

CSSPresentationModel has been introduced in order to
- manage source mappings,
- store live locations,
- perform the rawLocationToUILocation conversions.

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/297923002

git-svn-id: svn://svn.chromium.org/blink/trunk@179012 bbb929c8-8fbe-4397-9dbb-9b2b20218538