Always use global indexes for nodes and edges in the heap snapshot representation

- HeapSnapshotNode, HeapSnapshotEdge and HeapSnapshotRetainerEdge now store only reference to HeapSnapshot and global index of corresponding item.
- all index boundaries were moved out of these classes into corresponding iterators

BUG=None

Review URL: https://codereview.chromium.org/207523004

git-svn-id: svn://svn.chromium.org/blink/trunk@169732 bbb929c8-8fbe-4397-9dbb-9b2b20218538