mus: Implement interwindow drag and drop

services/ui/ws/server_window.cc

Index: services/ui/ws/server_window.cc
diff --git a/services/ui/ws/server_window.cc b/services/ui/ws/server_window.cc
index d2c781aa34f51411b81881f7983bdb20a916d25b..b36e944fde4915378538afb75c22d9f0065e2bf7 100644
--- a/services/ui/ws/server_window.cc
+++ b/services/ui/ws/server_window.cc
@@ -187,6 +187,10 @@ void ServerWindow::ClearHitTestMask() {
   hit_test_mask_.reset();
 }
 
+void ServerWindow::SetCanAcceptDrops(bool accepts_drops) {
+  accepts_drops_ = accepts_drops;
+}
+
 const ServerWindow* ServerWindow::GetRoot() const {
   return delegate_->GetRootWindow(this);
 }