Add GcRoot to clean up and enforce read barriers.

Introduce a value-type wrapper around Object* for GC roots so that 1)
we won't have to directly add the read barrier code in many places and
2) we can avoid accidentally bypassing/missing read barriers on GC
roots (the GcRoot interface ensures that the read barrier is executed
on a read).

The jdwp test passed.

Bug: 12687968
Change-Id: Ib167c7c325b3c7e3900133578815f04d219972a1
diff --git a/runtime/scoped_thread_state_change.h b/runtime/scoped_thread_state_change.h
index d691623..23aca45 100644
--- a/runtime/scoped_thread_state_change.h
+++ b/runtime/scoped_thread_state_change.h
@@ -19,6 +19,7 @@
 
 #include "base/casts.h"
 #include "jni_internal-inl.h"
+#include "read_barrier.h"
 #include "thread-inl.h"
 #include "verify_object.h"