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

ui/aura/client/focus_client.h

Index: ui/aura/client/focus_client.h
diff --git a/ui/aura/client/focus_client.h b/ui/aura/client/focus_client.h
index cec64ec6ddb84d72b5094b1c7c31d2053137f65c..f084349705c3e2c7f37d7e51f1e268c3bd572bea 100644
--- a/ui/aura/client/focus_client.h
+++ b/ui/aura/client/focus_client.h
@@ -6,10 +6,7 @@
 #define UI_AURA_CLIENT_FOCUS_CLIENT_H_
 
 #include "ui/aura/aura_export.h"
-
-namespace ui {
-class Event;
-}
+#include "ui/aura/window.h"
 
 namespace aura {
 class Window;
@@ -40,6 +37,9 @@
 AURA_EXPORT FocusClient* GetFocusClient(Window* window);
 AURA_EXPORT FocusClient* GetFocusClient(const Window* window);
 
+// The focus client is stored on the window using this property.
+AURA_EXPORT extern const WindowProperty<FocusClient*>* const kFocusClientKey;
+
 }  // namespace clients
 }  // namespace aura