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/thread_list.h b/runtime/thread_list.h
index 11f272c..7657fa8 100644
--- a/runtime/thread_list.h
+++ b/runtime/thread_list.h
@@ -31,10 +31,10 @@
 
 namespace art {
 namespace gc {
-  namespace collector {
-    class GarbageCollector;
-  }  // namespace collector
-  class GcPauseListener;
+namespace collector {
+class GarbageCollector;
+}  // namespace collector
+class GcPauseListener;
 }  // namespace gc
 class Closure;
 class RootVisitor;