observatory: populate 'architecture' in VM, and add numerical vmId in Class.

Knowing the word size and having an efficient (non-String) representation for class ids is important for the upcoming heap serialization features.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org//773303003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42115 260f80e4-7a28-3924-810f-c04153c831b5