Deprecate HeapSnapshot type

There is only one type of heap snapshot - kFull and we are not going to add any new types.

BUG=None

Review URL: https://codereview.chromium.org/12943004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14005 ce2b1a6d-e550-0410-aec6-3dcde31c8c00