DevTools: Unify handling of source ranges

This patch unifies the way of handling offset-based text ranges
in the source code (ad-hoc records and WebInspector.SearchResult) by
introducing WebInspector.SourceRange and migrating all clients.

R=aandrey, vsevik

Review URL: https://codereview.chromium.org/94973002

git-svn-id: svn://svn.chromium.org/blink/trunk@162872 bbb929c8-8fbe-4397-9dbb-9b2b20218538