Simplify instance reference offsets.

Don't encode Object's class. Use trailing rather than leading zeroes to give
offset position.

Change-Id: I1ae74e7a01f63696429644adf81cdf6ee58832fe
diff --git a/runtime/mirror/object.h b/runtime/mirror/object.h
index b68aef9..4ed96fc 100644
--- a/runtime/mirror/object.h
+++ b/runtime/mirror/object.h
@@ -17,6 +17,7 @@
 #ifndef ART_RUNTIME_MIRROR_OBJECT_H_
 #define ART_RUNTIME_MIRROR_OBJECT_H_
 
+#include "globals.h"
 #include "object_reference.h"
 #include "offsets.h"
 #include "verify_object.h"
@@ -60,6 +61,9 @@
 // Checks that we don't do field assignments which violate the typing system.
 static constexpr bool kCheckFieldAssignments = false;
 
+// Size of Object.
+static constexpr uint32_t kObjectHeaderSize = kUseBrooksReadBarrier ? 16 : 8;
+
 // C++ mirror of java.lang.Object
 class MANAGED LOCKABLE Object {
  public: