Add tools/ddbg_service.

This is the directory where we will work on a version of ddbg that
uses the vm service.

Not yet functional.

BUG=
R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org//575853002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@40334 260f80e4-7a28-3924-810f-c04153c831b5