Add presubmit check for scoped_ptr usage.

ui/wm/core/nested_accelerator_dispatcher_linux.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 "ui/wm/core/nested_accelerator_dispatcher.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "base/run_loop.h"
 #include "ui/base/accelerators/accelerator.h"
 #include "ui/events/event.h"
@@ skipping 20 matching lines @@
 bool IsKeyEvent(const XEvent* xev) {
   return xev->type == KeyPress || xev->type == KeyRelease;
 }
 #else
 #error Unknown build platform: you should have either use_ozone or use_x11.
 #endif
 
 scoped_ptr<ui::ScopedEventDispatcher> OverrideDispatcher(
     ui::PlatformEventDispatcher* dispatcher) {
   ui::PlatformEventSource* source = ui::PlatformEventSource::GetInstance();
-  return source ? source->OverrideDispatcher(dispatcher)
-                : scoped_ptr<ui::ScopedEventDispatcher>();
+  return source ? source->OverrideDispatcher(dispatcher) : nullptr;
 }
 
 }  // namespace
 
 class NestedAcceleratorDispatcherLinux : public NestedAcceleratorDispatcher,
                                          public ui::PlatformEventDispatcher {
  public:
   explicit NestedAcceleratorDispatcherLinux(NestedAcceleratorDelegate* delegate)
       : NestedAcceleratorDispatcher(delegate),
         restore_dispatcher_(OverrideDispatcher(this)) {}
@@ skipping 41 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(NestedAcceleratorDispatcherLinux);
 };
 
 scoped_ptr<NestedAcceleratorDispatcher> NestedAcceleratorDispatcher::Create(
     NestedAcceleratorDelegate* delegate,
     base::MessagePumpDispatcher* nested_dispatcher) {
   return make_scoped_ptr(new NestedAcceleratorDispatcherLinux(delegate));
 }
 
 }  // namespace wm