mus: Implement interwindow drag and drop

services/ui/ws/default_access_policy.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/ws/default_access_policy.h"
 
 #include "services/ui/ws/access_policy_delegate.h"
 #include "services/ui/ws/server_window.h"
 
 namespace ui {
@@ skipping 131 matching lines @@
 bool DefaultAccessPolicy::CanSetClientArea(const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
 
 bool DefaultAccessPolicy::CanSetHitTestMask(const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
 
+bool DefaultAccessPolicy::CanSetAcceptDrops(const ServerWindow* window) const {
+  return (WasCreatedByThisClient(window) &&
+          !delegate_->IsWindowRootOfAnotherTreeForAccessPolicy(window)) ||
+         delegate_->HasRootForAccessPolicy(window);
+}
+
 bool DefaultAccessPolicy::CanSetAcceptEvents(const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
 
 bool DefaultAccessPolicy::CanSetCursorProperties(
     const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||
          delegate_->HasRootForAccessPolicy(window);
 }
@@ skipping 42 matching lines @@
 bool DefaultAccessPolicy::IsValidIdForNewWindow(
     const ClientWindowId& id) const {
   // Clients using DefaultAccessPolicy only see windows they have created (for
   // the embed point they choose the id), so it's ok for clients to use whatever
   // id they want.
   return true;
 }
 
 }  // namespace ws
 }  // namespace ui