Change FocusSynchronizer to maintain active focus client and window.

ui/aura/client/aura_constants.h

Index: ui/aura/client/aura_constants.h
diff --git a/ui/aura/client/aura_constants.h b/ui/aura/client/aura_constants.h
index 145188866833e43d3df3cd35722d5fb0a2e23642..bcfab7f07854d67c7c44d4517d03e44d6747643a 100644
--- a/ui/aura/client/aura_constants.h
+++ b/ui/aura/client/aura_constants.h
@@ -21,6 +21,7 @@ enum class WindowType;
 
 namespace aura {
 namespace client {
+class FocusClient;
 
 enum class WindowEmbedType {
   NONE,
@@ -62,6 +63,9 @@ AURA_EXPORT extern const WindowProperty<bool>* const kCreatedByUserGesture;
 // attention.
 AURA_EXPORT extern const WindowProperty<bool>* const kDrawAttentionKey;
 
+// A property key to store the focus client on the window.
+AURA_EXPORT extern const WindowProperty<FocusClient*>* const kFocusClientKey;
+
 // A property key to store the host window of a window. This lets
 // WebContentsViews find the windows that should constrain NPAPI plugins.
 AURA_EXPORT extern const WindowProperty<Window*>* const kHostWindowKey;