Add read barriers for the roots in Runtime.

Bug: 12687968
Change-Id: If26518a8251702cfe4d5cd7d1f50e80e342704cf
diff --git a/runtime/read_barrier_c.h b/runtime/read_barrier_c.h
index f4af61f..1385c60 100644
--- a/runtime/read_barrier_c.h
+++ b/runtime/read_barrier_c.h
@@ -35,4 +35,9 @@
 #error "Only one of Baker or Brooks can be enabled at a time."
 #endif
 
+// A placeholder marker to indicate places to add read barriers in the
+// assembly code. This is a development time aid and to be removed
+// after read barriers are added.
+#define THIS_LOAD_REQUIRES_READ_BARRIER
+
 #endif  // ART_RUNTIME_READ_BARRIER_C_H_