base: Erase unused FOR_EACH_OBSERVER macro.

base/observer_list_unittest.cc

Index: base/observer_list_unittest.cc
diff --git a/base/observer_list_unittest.cc b/base/observer_list_unittest.cc
index 33f0f3285cd022fa4e879d51e4c5b99b1749787b..c1ab1d086257fd284100e1ac77369a7b3fa75a74 100644
--- a/base/observer_list_unittest.cc
+++ b/base/observer_list_unittest.cc
@@ -606,7 +606,8 @@ TEST(ObserverListTest, IteratorOutlivesList) {
   ListDestructor a(observer_list);
   observer_list->AddObserver(&a);
 
-  FOR_EACH_OBSERVER(Foo, *observer_list, Observe(0));
+  for (auto& observer : *observer_list)
+    observer.Observe(0);
   // If this test fails, there'll be Valgrind errors when this function goes out
   // of scope.
 }