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/mirror/art_method-inl.h b/runtime/mirror/art_method-inl.h
index 01b05a6..73de683 100644
--- a/runtime/mirror/art_method-inl.h
+++ b/runtime/mirror/art_method-inl.h
@@ -41,9 +41,8 @@
 
 template<ReadBarrierOption kReadBarrierOption>
 inline Class* ArtMethod::GetJavaLangReflectArtMethod() {
-  DCHECK(java_lang_reflect_ArtMethod_ != nullptr);
-  return ReadBarrier::BarrierForRoot<mirror::Class, kReadBarrierOption>(
-      &java_lang_reflect_ArtMethod_);
+  DCHECK(!java_lang_reflect_ArtMethod_.IsNull());
+  return java_lang_reflect_ArtMethod_.Read<kReadBarrierOption>();
 }
 
 inline Class* ArtMethod::GetDeclaringClass() {