Add tests for GlobalHandles::IterateObjectGroups.

src/global-handles.h

Index: src/global-handles.h
diff --git a/src/global-handles.h b/src/global-handles.h
index ffec6c20445f4abe9258b5c95024d17e2e9ea7f1..d4041ae394a2b381dd754e0859e69809990b707b 100644
--- a/src/global-handles.h
+++ b/src/global-handles.h
@@ -32,10 +32,15 @@
 #include "../include/v8-profiler.h"
 
 #include "list.h"
+#include "v8utils.h"
 
 namespace v8 {
 namespace internal {
 
+class GCTracer;
+class HeapStats;
+class ObjectVisitor;
+
 // Structure for tracking global handles.
 // A single list keeps all the allocated global handles.
 // Destroyed handles stay in the list but is added to the free list.