mus: Implement interwindow drag and drop

services/ui/ws/default_access_policy.cc

Index: services/ui/ws/default_access_policy.cc
diff --git a/services/ui/ws/default_access_policy.cc b/services/ui/ws/default_access_policy.cc
index 5d976599cb9dda110dff72f75578a0cb68cafc5e..7f6edcb34abcc3c04453ea845e2634fac10cde50 100644
--- a/services/ui/ws/default_access_policy.cc
+++ b/services/ui/ws/default_access_policy.cc
@@ -149,6 +149,12 @@ bool DefaultAccessPolicy::CanSetHitTestMask(const ServerWindow* window) const {
          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);