Access singletons with a new GetInstance() method instead of Singleton<T>.

app/active_window_watcher_x.cc

Index: app/active_window_watcher_x.cc
diff --git a/app/active_window_watcher_x.cc b/app/active_window_watcher_x.cc
index 9b861e65e9522e557d1dda0bb9f38e07c0b022fb..be47c432c9c7ac02798abec577448b691797a164 100644
--- a/app/active_window_watcher_x.cc
+++ b/app/active_window_watcher_x.cc
@@ -11,13 +11,18 @@
 static Atom kNetActiveWindowAtom = None;
 
 // static
+ActiveWindowWatcherX* ActiveWindowWatcherX::GetInstance() {
+  return Singleton<ActiveWindowWatcherX>::get();
+}
+
+// static
 void ActiveWindowWatcherX::AddObserver(Observer* observer) {
-  Singleton<ActiveWindowWatcherX>::get()->observers_.AddObserver(observer);
+  ActiveWindowWatcherX::GetInstance()->observers_.AddObserver(observer);
 }
 
 // static
 void ActiveWindowWatcherX::RemoveObserver(Observer* observer) {
-  Singleton<ActiveWindowWatcherX>::get()->observers_.RemoveObserver(observer);
+  ActiveWindowWatcherX::GetInstance()->observers_.RemoveObserver(observer);
 }
 
 ActiveWindowWatcherX::ActiveWindowWatcherX() {