ART: More header cleanups

Pull out more dependencies through forward declarations.

Test: m test-art-host
Change-Id: I7d86726928937f788b956ec9eac91532d66d57ae
diff --git a/runtime/monitor.h b/runtime/monitor.h
index 6dc706f..96c5a5b 100644
--- a/runtime/monitor.h
+++ b/runtime/monitor.h
@@ -30,13 +30,13 @@
 #include "base/mutex.h"
 #include "gc_root.h"
 #include "lock_word.h"
-#include "object_callbacks.h"
 #include "read_barrier_option.h"
 #include "thread_state.h"
 
 namespace art {
 
 class ArtMethod;
+class IsMarkedVisitor;
 class LockWord;
 template<class T> class Handle;
 class StackVisitor;