Remove FOR_EACH_OBSERVER from arc/ since it's being deprecated

components/arc/instance_holder.h

Index: components/arc/instance_holder.h
diff --git a/components/arc/instance_holder.h b/components/arc/instance_holder.h
index 0edb0f2c9df0cf7438872a5503aebd1ac56da17b..8455814581c61997f4d4ec602c6373cdbd9bb41b 100644
--- a/components/arc/instance_holder.h
+++ b/components/arc/instance_holder.h
@@ -99,9 +99,11 @@ class InstanceHolder {
     instance_ = instance;
     version_ = version;
     if (instance_) {
-      FOR_EACH_OBSERVER(Observer, observer_list_, OnInstanceReady());
+      for (auto& observer : observer_list_)
+        observer.OnInstanceReady();
     } else {
-      FOR_EACH_OBSERVER(Observer, observer_list_, OnInstanceClosed());
+      for (auto& observer : observer_list_)
+        observer.OnInstanceClosed();
     }
   }