Improve mark-compact object grouping interface.

Improve mark-compact object grouping interface.

The main goal was to improve O(n^2) behavior when there are many object groups.  The old API required the grouping to be done on the v8 side, along with a linear search.  The new interface requires the caller to do the grouping, passing V8 entire groups at a time.  This removes the group id concept on the v8 side.

  - Changed AddObjectToGroup to AddObjectGroup.
  - Removed the group id concept from the V8 side.
  - Remove a static constructor while I'm here, lazily initialize
    the object groups list.
  - Cleaned up return by non-const references to return pointers.

Review URL: http://codereview.chromium.org/13341

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@965 ce2b1a6d-e550-0410-aec6-3dcde31c8c00