Add HeapProfiler::GetPersistentHandleCount to be able to track the number of persistent handles

src/global-handles.h

Index: src/global-handles.h
===================================================================
--- src/global-handles.h	(revision 10957)
+++ src/global-handles.h	(working copy)
@@ -143,6 +143,11 @@
     return number_of_global_object_weak_handles_;
   }
 
+  // Returns the current number of handles to global objects.
+  int NumberOfGlobalHandles() {
+    return number_of_global_handles_;
+  }
+
   // Clear the weakness of a global handle.
   void ClearWeakness(Object** location);
 
@@ -248,6 +253,9 @@
   // number_of_weak_handles_.
   int number_of_global_object_weak_handles_;
 
+  // Field always containing the number of handles to global objects.
+  int number_of_global_handles_;
+
   // List of all allocated node blocks.
   NodeBlock* first_block_;