SVGUseElement::invalidateShadowTree() should not check confusingAndOftenMisusedAttached()

This looks like it's trying to check inActiveDocument().

Review URL: https://codereview.chromium.org/51883012

git-svn-id: svn://svn.chromium.org/blink/trunk@161019 bbb929c8-8fbe-4397-9dbb-9b2b20218538