DevTools: remove poisonous CSS selector.

*::before, *::after poisons CSS matcher cache (authoring bit), so every
time node is attached to DOM, all parent nodes are scanned.

See also:
  https://codereview.chromium.org/177003018

BUG=

Review URL: https://codereview.chromium.org/388153003

git-svn-id: svn://svn.chromium.org/blink/trunk@178165 bbb929c8-8fbe-4397-9dbb-9b2b20218538