aura/ash cleanup: Use a single window-key for constrained windows.

ash/ash_constants.h

Index: ash/ash_constants.h
diff --git a/ash/ash_constants.h b/ash/ash_constants.h
index 8e889851ea668db4fe70d0c74dac9fd7dba34aae..cad4354bcb0843c62d1fd7a6d68d3bdcbfd53730 100644
--- a/ash/ash_constants.h
+++ b/ash/ash_constants.h
@@ -13,11 +13,6 @@ typedef unsigned int SkColor;
 
 namespace ash {
 
-// The window is a constrained window and lives therefore entirely within
-// another aura window.
-ASH_EXPORT extern const aura::WindowProperty<bool>* const
-    kConstrainedWindowKey;
-
 // In the window corners, the resize areas don't actually expand bigger, but the
 // 16 px at the end of each edge triggers diagonal resizing.
 ASH_EXPORT extern const int kResizeAreaCornerSize;