Rename object_bitmap to heap_bitmap (since that's what the class is called).

Change-Id: Idce6e9062545eb13b701e6b7e371c262977814d1
diff --git a/src/mark_sweep.h b/src/mark_sweep.h
index bcb4df0..8c37456 100644
--- a/src/mark_sweep.h
+++ b/src/mark_sweep.h
@@ -5,7 +5,7 @@
 
 #include "macros.h"
 #include "mark_stack.h"
-#include "object_bitmap.h"
+#include "heap_bitmap.h"
 #include "offsets.h"
 
 namespace art {