Use ATTRIBUTE_UNUSED more.

Use it in lieu of UNUSED(), which had some incorrect uses.

Change-Id: If247dce58b72056f6eea84968e7196f0b5bef4da
diff --git a/runtime/mirror/array-inl.h b/runtime/mirror/array-inl.h
index 3d54029..ec7d758 100644
--- a/runtime/mirror/array-inl.h
+++ b/runtime/mirror/array-inl.h
@@ -100,9 +100,8 @@
   explicit SetLengthVisitor(int32_t length) : length_(length) {
   }
 
-  void operator()(Object* obj, size_t usable_size) const
+  void operator()(Object* obj, size_t usable_size ATTRIBUTE_UNUSED) const
       SHARED_REQUIRES(Locks::mutator_lock_) {
-    UNUSED(usable_size);
     // Avoid AsArray as object is not yet in live bitmap or allocation stack.
     Array* array = down_cast<Array*>(obj);
     // DCHECK(array->IsArrayInstance());