mus: Implement interwindow drag and drop

services/ui/ws/server_window.h

Index: services/ui/ws/server_window.h
diff --git a/services/ui/ws/server_window.h b/services/ui/ws/server_window.h
index 29b770ed1a77c48b07889289487feecd118e0675..27373c17104a65a148ae187537ad6dd25f6f999a 100644
--- a/services/ui/ws/server_window.h
+++ b/services/ui/ws/server_window.h
@@ -86,6 +86,9 @@ class ServerWindow {
   void SetHitTestMask(const gfx::Rect& mask);
   void ClearHitTestMask();
 
+  bool can_accept_drags() const { return accepts_drags_; }
+  void SetCanAcceptDrags(bool accepts_drags);
+
   int32_t cursor() const { return static_cast<int32_t>(cursor_id_); }
   int32_t non_client_cursor() const {
     return static_cast<int32_t>(non_client_cursor_id_);
@@ -241,6 +244,10 @@ class ServerWindow {
   // means all events miss the window. If null there is no mask.
   std::unique_ptr<gfx::Rect> hit_test_mask_;
 
+  // Whether this window can be the target in a drag and drop
+  // operation. Clients must opt-in to this.
+  bool accepts_drags_ = false;
+
   base::ObserverList<ServerWindowObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ServerWindow);