Revert of 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 74ad3e09c8db15283e45a3f2a3818bd6027788a6..b211315fc9ca0c97d991b01993559d1fe0cd4d51 100644
--- a/ui/aura/mus/focus_synchronizer.cc
+++ b/ui/aura/mus/focus_synchronizer.cc
@@ -16,16 +16,13 @@
 
 FocusSynchronizer::FocusSynchronizer(FocusSynchronizerDelegate* delegate,
                                      ui::mojom::WindowTree* window_tree)
-    : delegate_(delegate), window_tree_(window_tree), env_(Env::GetInstance()) {
-  DCHECK(env_);
-  env_->AddObserver(this);
+    : delegate_(delegate), window_tree_(window_tree) {
+  Env::GetInstance()->AddObserver(this);
 }
 
 FocusSynchronizer::~FocusSynchronizer() {
-  if (env_) {
-    SetActiveFocusClient(nullptr);
-    env_->RemoveObserver(this);
-  }
+  SetActiveFocusClient(nullptr);
+  Env::GetInstance()->RemoveObserver(this);
 }
 
 void FocusSynchronizer::SetFocusFromServer(WindowMus* window) {
@@ -84,13 +81,6 @@
 
 void FocusSynchronizer::OnWindowInitialized(Window* window) {}
 
-void FocusSynchronizer::OnWillDestroyEnv() {
-  // TODO(sky): this override should not be necessary. http://crbug.com/674803.
-  SetActiveFocusClient(nullptr);
-  env_->RemoveObserver(this);
-  env_ = nullptr;
-}
-
 void FocusSynchronizer::OnActiveFocusClientChanged(
     client::FocusClient* focus_client,
     Window* window) {