Disable the support for pseudo-classes in matching criteria of content insertion points in author shadow trees.

This makes the implementation match the current spec. To support
<details>/<summary> elements, pseudo-classes are still supported in
UA shadow trees.

BUG=343332

Review URL: https://codereview.chromium.org/215103005

git-svn-id: svn://svn.chromium.org/blink/trunk@171068 bbb929c8-8fbe-4397-9dbb-9b2b20218538