DevTools: Introduce setHighlighterType on SourceFrame to allow setting file format explicitly.

R=pfeldman

Review URL: https://chromiumcodereview.appspot.com/25002003

git-svn-id: svn://svn.chromium.org/blink/trunk@158446 bbb929c8-8fbe-4397-9dbb-9b2b20218538