Make pseudo-classes selector queries work on DocumentFragment

Make pseudo-classes selector queries work on DocumentFragment as well. This CL
extends tasak@'s r167322 to cover all DocumentFragment nodes, and not only the
ShadowRoot. This was made easier by esprehn@'s r169787 which made the
corresponding code more shareable.

The new behavior is consistent with Firefox 28.

R=esprehn@chromium.org, dglazkov@chromium.org
BUG=355386
TEST=fast/dom/DocumentFragment/sibling-rules-under-document-fragment.html

Review URL: https://codereview.chromium.org/270503006

git-svn-id: svn://svn.chromium.org/blink/trunk@174095 bbb929c8-8fbe-4397-9dbb-9b2b20218538