mus: Implement interwindow drag and drop

services/ui/ws/access_policy.h

Index: services/ui/ws/access_policy.h
diff --git a/services/ui/ws/access_policy.h b/services/ui/ws/access_policy.h
index 452f17a9610925d1aa91d1f8d7ea07f9673b7b32..a8e25e2a571550e12675f2ac12374e1d1ebd6560 100644
--- a/services/ui/ws/access_policy.h
+++ b/services/ui/ws/access_policy.h
@@ -58,10 +58,12 @@ class AccessPolicy {
   virtual bool CanSetFocus(const ServerWindow* window) const = 0;
   virtual bool CanSetClientArea(const ServerWindow* window) const = 0;
   virtual bool CanSetHitTestMask(const ServerWindow* window) const = 0;
+  virtual bool CanSetAcceptDrops(const ServerWindow* window) const = 0;
   virtual bool CanSetAcceptEvents(const ServerWindow* window) const = 0;
   // Used for all client controllable cursor properties; which cursor should be
   // displayed, visibility, locking, etc.
   virtual bool CanSetCursorProperties(const ServerWindow* window) const = 0;
+  virtual bool CanInitiateDragLoop(const ServerWindow* window) const = 0;
   virtual bool CanInitiateMoveLoop(const ServerWindow* window) const = 0;
 
   // Returns whether the client should notify on a hierarchy change.