Fix HeapSnapshotsDiff test, diff implementation, and a bug introduced

Fix HeapSnapshotsDiff test, diff implementation, and a bug introduced
during snapshot size optimization.

Sorry, now I figured out that the diff implementation itself was also
incorrect.  Reachable nodes must be filtered from the beginning,
otherwise, an object that is already disconnected, but not discarded
yet, will not appear as a deleted (thankfully, this bug for some
reason had appeared on the x64 port.)

BUG=868
TEST=HeapSnapshotRootPreservedAfterSorting

Review URL: http://codereview.chromium.org/3531005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5570 ce2b1a6d-e550-0410-aec6-3dcde31c8c00