DevTools: Use columnNumber when showing breakpoint snippets in sidebar pane.

Drive-by: Move code for splitting string by lines to utilities.

R=pfeldman@chromium.org, pfeldman

Review URL: https://codereview.chromium.org/151453002

git-svn-id: svn://svn.chromium.org/blink/trunk@166307 bbb929c8-8fbe-4397-9dbb-9b2b20218538