cpplint: Cleanup errors

Cleanup errors from upstream cpplint in preparation
for moving art's cpplint fork to upstream tip-of-tree cpplint.

Test: cd art && mm
Bug: 68951293
Change-Id: I15faed4594cbcb8399850f8bdee39d42c0c5b956
diff --git a/runtime/read_barrier.h b/runtime/read_barrier.h
index 00674b2..d4b9f43 100644
--- a/runtime/read_barrier.h
+++ b/runtime/read_barrier.h
@@ -28,8 +28,8 @@
 
 namespace art {
 namespace mirror {
-  class Object;
-  template<typename MirrorType> class HeapReference;
+class Object;
+template<typename MirrorType> class HeapReference;
 }  // namespace mirror
 class ArtMethod;