DevTools: Treat system object distances with less priority

Currently if a user object is retained by both a system-space object
(e.g. a debugger) and another user object, the system object might be
shown earlier in the retainers tree. This happens if its distance is
smaller than distances of other retaining user objects.

The patch treats links from system-space objects to user objects with
less priority, so these links are shown at the bottom of the retainers
tree.

Review URL: https://codereview.chromium.org/222443003

git-svn-id: svn://svn.chromium.org/blink/trunk@170765 bbb929c8-8fbe-4397-9dbb-9b2b20218538