Add back in isRoot.

This method got used in the time that it took
the CQ to land the patch. :(

TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/235703008

git-svn-id: svn://svn.chromium.org/blink/trunk@171495 bbb929c8-8fbe-4397-9dbb-9b2b20218538