Fix issue in which tagQName could be accessed without a tag selector.
BUG=340697
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/154233002

git-svn-id: svn://svn.chromium.org/blink/trunk@166439 bbb929c8-8fbe-4397-9dbb-9b2b20218538