More code for the read barrier support.

Make it possible to disable the RB in Object::SizeOf() (and the
functions it calls transitively) which the collector will need to call
to get the size of an object when copying.

Add Object::AtomicSetReadBarrierPointer() for atomic write of a RB
pointer.

Bug: 12687968
Change-Id: Ibedd252860ac7ccd17e4e7d71b377a8892b48ff0
diff --git a/runtime/mirror/art_method.h b/runtime/mirror/art_method.h
index fd5ac19..f513a04 100644
--- a/runtime/mirror/art_method.h
+++ b/runtime/mirror/art_method.h
@@ -423,7 +423,9 @@
 
   static void SetClass(Class* java_lang_reflect_ArtMethod);
 
+  template <bool kDoReadBarrier = true>
   static Class* GetJavaLangReflectArtMethod() {
+    // This does not need a RB because it is a root.
     return java_lang_reflect_ArtMethod_;
   }