Debugger: do not expose global object.

src/runtime/runtime-debug.cc

Index: src/runtime/runtime-debug.cc
diff --git a/src/runtime/runtime-debug.cc b/src/runtime/runtime-debug.cc
index fcce24ff8ea4e559a9a4ed7156c0c990df4280ad..97c3ff48a9a6535208c91ceb733e3f7ae3fa5af4 100644
--- a/src/runtime/runtime-debug.cc
+++ b/src/runtime/runtime-debug.cc
@@ -1399,6 +1399,11 @@ static int DebugReferencedBy(HeapIterator* iterator, JSObject* target,
           }
         }
 
+        // Do not expose the global object directly.
+        if (obj->IsJSGlobalObject()) {
+          obj = JSGlobalObject::cast(obj)->global_proxy();
+        }
+
         if (obj != NULL) {
           // Valid reference found add to instance array if supplied an update
           // count.