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
Geolocation: delete ancient fixme poiting to a closed issue.

BUG=None

Review URL: https://codereview.chromium.org/222513002

git-svn-id: svn://svn.chromium.org/blink/trunk@170766 bbb929c8-8fbe-4397-9dbb-9b2b20218538