ART: More header cleanups

Pull out more dependencies through forward declarations.

Test: m test-art-host
Change-Id: I7d86726928937f788b956ec9eac91532d66d57ae
diff --git a/runtime/gc/reference_processor.h b/runtime/gc/reference_processor.h
index 38b68cb..a8135d9 100644
--- a/runtime/gc/reference_processor.h
+++ b/runtime/gc/reference_processor.h
@@ -20,11 +20,11 @@
 #include "base/mutex.h"
 #include "globals.h"
 #include "jni.h"
-#include "object_callbacks.h"
 #include "reference_queue.h"
 
 namespace art {
 
+class IsMarkedVisitor;
 class TimingLogger;
 
 namespace mirror {