Drop all uses of //base except ObserverList from view mgr client lib

mojo/services/view_manager/public/cpp/lib/view_observer.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "view_manager/public/cpp/view_observer.h"
 
-#include "base/basictypes.h"
-
 namespace mojo {
 
 ////////////////////////////////////////////////////////////////////////////////
 // ViewObserver, public:
 
 ViewObserver::TreeChangeParams::TreeChangeParams()
-    : target(NULL),
-      old_parent(NULL),
-      new_parent(NULL),
-      receiver(NULL) {}
+    : target(nullptr),
+      old_parent(nullptr),
+      new_parent(nullptr),
+      receiver(nullptr) {
+}
 
 }  // namespace mojo