DevTools: [CodeMirror] respect CRLF line endings

This patch adds a functionality to respect CRLF line endings in
CodeMirrorTextEditor; this functionality was initially missing in it.
A test to verify this behaviour is also brought in.

BUG=263364
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/19540027

git-svn-id: svn://svn.chromium.org/blink/trunk@154760 bbb929c8-8fbe-4397-9dbb-9b2b20218538