DevTools: slight tweaking of step-into-selection feature

This patch does a slight CSS tweaking related to step-into-selection
feature.

BUG=275661

Review URL: https://chromiumcodereview.appspot.com/23684038

git-svn-id: svn://svn.chromium.org/blink/trunk@157527 bbb929c8-8fbe-4397-9dbb-9b2b20218538