Add a comment on ReadBarrier::Mark() about ALWAYS_INLINE.

This is a follow up on CL 199360.

Bug: 12687968
Bug: 26744236
Change-Id: I9930894a403e897b85d77caf92834afaa9a860a0
diff --git a/runtime/read_barrier.h b/runtime/read_barrier.h
index 77be6cf..b7bd99b 100644
--- a/runtime/read_barrier.h
+++ b/runtime/read_barrier.h
@@ -80,6 +80,7 @@
   static void AssertToSpaceInvariant(GcRootSource* gc_root_source, mirror::Object* ref)
       SHARED_REQUIRES(Locks::mutator_lock_);
 
+  // ALWAYS_INLINE on this caused a performance regression b/26744236.
   static mirror::Object* Mark(mirror::Object* obj) SHARED_REQUIRES(Locks::mutator_lock_);
 
   static mirror::Object* WhitePtr() {