DevTools: Freshen up popover styles.

Drop the aging yellow popover and match the tooltip style in Chrome.

TBR=sergeyv
BUG=181703

Review URL: https://codereview.chromium.org/401013002

git-svn-id: svn://svn.chromium.org/blink/trunk@178466 bbb929c8-8fbe-4397-9dbb-9b2b20218538