[DevTools] Fix inspector overlay page visual glitches.

Turned out that changing z-index on :active is not working properly.

BUG=357547,326416

Review URL: https://codereview.chromium.org/216883006

git-svn-id: svn://svn.chromium.org/blink/trunk@170344 bbb929c8-8fbe-4397-9dbb-9b2b20218538