Use isDocumentFragment() instead of nodeType() in collectChildrenAndRemoveFromOldParent()

isDocumentFragment() is a fast bitfield check while nodeType() is virtual.
This saves one virtual call per child removal (not much, but why not?).

Review URL: https://codereview.chromium.org/63373004

git-svn-id: svn://svn.chromium.org/blink/trunk@161691 bbb929c8-8fbe-4397-9dbb-9b2b20218538