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_test.cc b/runtime/mirror/object_test.cc
index 7fa664d..1aeba74 100644
--- a/runtime/mirror/object_test.cc
+++ b/runtime/mirror/object_test.cc
@@ -76,7 +76,8 @@
 
 // Keep constants in sync.
 TEST_F(ObjectTest, Constants) {
-  EXPECT_EQ(kObjectReferenceSize, sizeof(mirror::HeapReference<mirror::Object>));
+  EXPECT_EQ(kObjectReferenceSize, sizeof(HeapReference<Object>));
+  EXPECT_EQ(kObjectHeaderSize, sizeof(Object));
 }
 
 // Keep the assembly code constats in sync.