Fix HashSet misuse in SVGElementInstance::detach()

SVGElementInstance::detach() passes empty values to m_element->instancesForElement()
(HashSet) which is not supported by WTF::HashSet and makes svg layout tests fail when
https://codereview.chromium.org/226003005/ is applied.

Review URL: https://codereview.chromium.org/263833017

git-svn-id: svn://svn.chromium.org/blink/trunk@173393 bbb929c8-8fbe-4397-9dbb-9b2b20218538