Change reference processing to use heap references.

Removes several SetReferents for updating moved referents. Cleaned
up other aspects of the code.

Change-Id: Ibcb4d713fadea617efee7e936352ddf77ff4c370
diff --git a/runtime/gc/collector/mark_sweep.h b/runtime/gc/collector/mark_sweep.h
index a0a0dd8..a44d8a1 100644
--- a/runtime/gc/collector/mark_sweep.h
+++ b/runtime/gc/collector/mark_sweep.h
@@ -178,6 +178,10 @@
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
       EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_);
 
+  static bool HeapReferenceMarkedCallback(mirror::HeapReference<mirror::Object>* ref, void* arg)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
+      EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_);
+
   static void MarkRootCallback(mirror::Object** root, void* arg, uint32_t thread_id,
                                RootType root_type)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)