In order object graph traversal for image writing

Rather than allocation order an in order traversal of the object graph
can improve locality such as between a String and its char[].

Change-Id: I9295122b13362a550bdb5161c6c7a8d29632712a
diff --git a/src/heap_bitmap.h b/src/heap_bitmap.h
index aa542db..70684b9 100644
--- a/src/heap_bitmap.h
+++ b/src/heap_bitmap.h
@@ -90,6 +90,8 @@
 
   void Walk(Callback* callback, void* arg);
 
+  void InOrderWalk(HeapBitmap::Callback* callback, void* arg);
+
   void ScanWalk(uintptr_t base, uintptr_t max, ScanCallback* thunk, void* arg);
 
   static void SweepWalk(const HeapBitmap& live,