DevTools: Allow large retained sizes in heap snapshot.

Also removed orphan nodes tests since we actually do not
support snapshots with orphan nodes as dominator tree building
algorithm fails for these.

BUG=

Review URL: https://codereview.chromium.org/164083002

git-svn-id: svn://svn.chromium.org/blink/trunk@167200 bbb929c8-8fbe-4397-9dbb-9b2b20218538