DevTools: Fix big font in Sources tabbed pane chevron dropdown menu.

BUG=262018
R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/19934004

git-svn-id: svn://svn.chromium.org/blink/trunk@154657 bbb929c8-8fbe-4397-9dbb-9b2b20218538