Add Node::isInDocumentTree()

The behavior of Node::isInDocumentTree() is equal to "in a document tree" defined in
DOM Standard: https://dom.spec.whatwg.org/#in-a-document-tree

Review-Url: https://codereview.chromium.org/2146973004
Cr-Commit-Position: refs/heads/master@{#405706}
4 files changed