selectorQuery should not throw DOM Exception 12 when unknow or custom pseudo elements are found.

BUG=273960
TEST=fast/dom/shadow/custom-pseudo-in-selector-api.html

Review URL: https://chromiumcodereview.appspot.com/23146011

git-svn-id: svn://svn.chromium.org/blink/trunk@156538 bbb929c8-8fbe-4397-9dbb-9b2b20218538