DevTools: [Media Inspector] do not show bottom border if inspector is empty

This patch removes one-pixel bottom border in media inspector which
appears to be excessive in case of empty media inspector.

R=pfeldman@chromium.org, dgozman, pfeldman

Review URL: https://codereview.chromium.org/339323003

git-svn-id: svn://svn.chromium.org/blink/trunk@176632 bbb929c8-8fbe-4397-9dbb-9b2b20218538