Switch ObserverList::size() to ObserverList::might_have_observers() Pt.1

chrome/browser/ui/panels/panel_mouse_watcher.cc

 // Copyright (c) 2011 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 "chrome/browser/ui/panels/panel_mouse_watcher.h"
 
 #include "chrome/browser/ui/panels/panel_mouse_watcher_observer.h"
 #include "ui/gfx/point.h"
 
 PanelMouseWatcher::PanelMouseWatcher() {
 }
 
 PanelMouseWatcher::~PanelMouseWatcher() {
 }
 
 void PanelMouseWatcher::AddObserver(PanelMouseWatcherObserver* observer) {
+  bool already_started = observers_.might_have_observers();
   observers_.AddObserver(observer);
-  if (observers_.size() == 1)
+  if (!already_started)
     Start();
 }
 
 void PanelMouseWatcher::RemoveObserver(PanelMouseWatcherObserver* observer) {
   DCHECK(observers_.HasObserver(observer));
   observers_.RemoveObserver(observer);
-  if (observers_.size() == 0)
+  if (!observers_.might_have_observers())
     Stop();
 }
 
 void PanelMouseWatcher::NotifyMouseMovement(const gfx::Point& mouse_position) {
   FOR_EACH_OBSERVER(PanelMouseWatcherObserver, observers_,
                     OnMouseMove(mouse_position));
 }