Implement Node.ownerScope

The ownerScope property is equivalent to walking up the parentNode
pointers until you hit the top and returning that node if it's a
Document or ShadowRoot.

This means that the ownerScope of ShadowRoot and Document is always
itself, and the ownerScope of an Element that is not the descendant
of a ShadowRoot, and is not in the document is null.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/810793005