DevTools: [MQI] text-shadow for label instead of black background color

This patch uses text shadow for label text instead of black half-opaque
background color.

BUG=382996

Review URL: https://codereview.chromium.org/351263002

git-svn-id: svn://svn.chromium.org/blink/trunk@177069 bbb929c8-8fbe-4397-9dbb-9b2b20218538