DevTools: [Popover] fix css for popover scrollbar

Fix reference to non-existent image in some rules to style popover
scrollbar.

R=vsevik, apavlov

Review URL: https://codereview.chromium.org/420713004

git-svn-id: svn://svn.chromium.org/blink/trunk@178933 bbb929c8-8fbe-4397-9dbb-9b2b20218538