Support a negation pseudo-class, :not(), in matching criteria of content insertion points.

The spec: http://w3c.github.io/webcomponents/spec/shadow/#satisfying-matching-criteria

BUG=370670

Review URL: https://codereview.chromium.org/279763002

git-svn-id: svn://svn.chromium.org/blink/trunk@174929 bbb929c8-8fbe-4397-9dbb-9b2b20218538