Converts chrome to aura-mus

ui/aura/mus/focus_synchronizer.cc

Index: ui/aura/mus/focus_synchronizer.cc
diff --git a/ui/aura/mus/focus_synchronizer.cc b/ui/aura/mus/focus_synchronizer.cc
index b211315fc9ca0c97d991b01993559d1fe0cd4d51..4c24ccc10316df90d0231170a12114e0d7d3448f 100644
--- a/ui/aura/mus/focus_synchronizer.cc
+++ b/ui/aura/mus/focus_synchronizer.cc
@@ -16,13 +16,16 @@ namespace aura {
 
 FocusSynchronizer::FocusSynchronizer(FocusSynchronizerDelegate* delegate,
                                      ui::mojom::WindowTree* window_tree)
-    : delegate_(delegate), window_tree_(window_tree) {
-  Env::GetInstance()->AddObserver(this);
+    : delegate_(delegate), window_tree_(window_tree), env_(Env::GetInstance()) {
+  DCHECK(env_);
+  env_->AddObserver(this);
 }
 
 FocusSynchronizer::~FocusSynchronizer() {
-  SetActiveFocusClient(nullptr);
-  Env::GetInstance()->RemoveObserver(this);
+  if (env_) {
+    SetActiveFocusClient(nullptr);
+    env_->RemoveObserver(this);
+  }
 }
 
 void FocusSynchronizer::SetFocusFromServer(WindowMus* window) {
@@ -81,6 +84,12 @@ void FocusSynchronizer::OnWindowFocused(Window* gained_focus,
 
 void FocusSynchronizer::OnWindowInitialized(Window* window) {}
 
+void FocusSynchronizer::OnWillDestroyEnv() {
+  SetActiveFocusClient(nullptr);
+  env_->RemoveObserver(this);
+  env_ = nullptr;
+}
+
 void FocusSynchronizer::OnActiveFocusClientChanged(
     client::FocusClient* focus_client,
     Window* window) {