Converts MaximizeModeWindowManager to use ash/common types

ash/mus/bridge/wm_window_mus.cc

Index: ash/mus/bridge/wm_window_mus.cc
diff --git a/ash/mus/bridge/wm_window_mus.cc b/ash/mus/bridge/wm_window_mus.cc
index 844a83a56043ea997d30ef08d2f0f3e0a4a9c7b6..e723ad35dd963e35cd813e45714e049059ea4542 100644
--- a/ash/mus/bridge/wm_window_mus.cc
+++ b/ash/mus/bridge/wm_window_mus.cc
@@ -715,6 +715,10 @@ void WmWindowMus::RemoveObserver(WmWindowObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
+bool WmWindowMus::HasObserver(const WmWindowObserver* observer) const {
+  return observers_.HasObserver(observer);
+}
+
 void WmWindowMus::OnTreeChanging(const TreeChangeParams& params) {
   WmWindowObserver::TreeChangeParams wm_params;
   wm_params.target = Get(params.target);