Remove bogus forward declaration of ObjectGroup.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/14333016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14425 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/v8globals.h

@@ -162,7 +162,6 @@ class Variable;
 class RelocInfo;
 class Deserializer;
 class MessageLocation;
-class ObjectGroup;
 class VirtualMemory;
 class Mutex;