HeapSnapshot: Sort by distance doesn't work properly for node children in Summary view.

BUG=285766
R=alph@chromium.org

Review URL: https://codereview.chromium.org/23494034

git-svn-id: svn://svn.chromium.org/blink/trunk@157453 bbb929c8-8fbe-4397-9dbb-9b2b20218538