Teach V8GCController how to traverse TemplateContentDocumentFragment::host

This allows us to remove the [KeepAttributeAliveForGC] extended IDL attribute,
at the cost of a ridiculously-named new method on Node.

R=haraken@chromium.org
BUG=312518

Review URL: https://codereview.chromium.org/48633006

git-svn-id: svn://svn.chromium.org/blink/trunk@161282 bbb929c8-8fbe-4397-9dbb-9b2b20218538