Presubmit check for DevTools should remove the generated protocol_externs.js file

R=yurys@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21538002

git-svn-id: svn://svn.chromium.org/blink/trunk@155349 bbb929c8-8fbe-4397-9dbb-9b2b20218538