Reland of Improves focus/activation for aura-mus and DesktopNativeWidgetAura

ui/aura/env_observer.h

Index: ui/aura/env_observer.h
diff --git a/ui/aura/env_observer.h b/ui/aura/env_observer.h
index 03fcf3bd171e3291bf310b5dd6630470a337b07a..274bf3f4bc48ac04561a4bc2b8dadca7c652d8f4 100644
--- a/ui/aura/env_observer.h
+++ b/ui/aura/env_observer.h
@@ -11,6 +11,10 @@
 
 class Window;
 class WindowTreeHost;
+
+namespace client {
+class FocusClient;
+}
 
 class AURA_EXPORT EnvObserver {
  public:
@@ -26,6 +30,10 @@
   // Called right before Env is destroyed.
   virtual void OnWillDestroyEnv() {}
 
+  // Called from Env::SetActiveFocusClient(), see it for details.
+  virtual void OnActiveFocusClientChanged(client::FocusClient* focus_client,
+                                          Window* window) {}
+
  protected:
   virtual ~EnvObserver() {}
 };