DevTools: Fix show retainers up to the root

Currently it stops showing retainers at the (GC roots) owned nodes after
their type has been changed to synthetic v8:r15492

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/19663009

git-svn-id: svn://svn.chromium.org/blink/trunk@154653 bbb929c8-8fbe-4397-9dbb-9b2b20218538