First shot at implementing drag&drop for Aura

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 4f13103098048dbf2ade50eaa1680d5131ffbaa4..180d0e8fe0175a84e1d5f2989dde94323645e4ca 100644
--- a/ui/aura/client/aura_constants.h
+++ b/ui/aura/client/aura_constants.h
@@ -29,6 +29,13 @@ AURA_EXPORT extern const char kTooltipTextKey[];
 // A property key to store the boolean property of window modality.
 AURA_EXPORT extern const char kModalKey[];
 
+// A property key to store the drag and drop client for the desktop. The type of
+// the value is |aura::DragDropClient*|.
+AURA_EXPORT extern const char kDesktopDragDropClientKey[];
+
+// A property key to store the drag and drop delegate for a window. The type of
+// the value is |aura::WindowDragDropDelegate*|.
+AURA_EXPORT extern const char kDragDropDelegateKey[];
 }  // namespace aura
 
 #endif  // UI_AURA_CLIENT_AURA_CONSTANTS_H_