Add initial version of retainers heap profile.

Add initial version of retainers heap profile.

The profile is taken together with constructors profile. In theory, it
should represent a complete heap graph. However, this takes a lot of memory,
so it is reduced to a more compact, but still useful form. Namely:

 - objects are aggregated by their constructors, except for Array and Object
   instances, that are too hetereogeneous;

 - for Arrays and Objects, initially every instance is concerned, but then
   they are grouped together based on their retainer graph paths similarity (e.g.
   if two objects has the same retainer, they are considered equal);

Review URL: http://codereview.chromium.org/200132

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2903 ce2b1a6d-e550-0410-aec6-3dcde31c8c00