mus: Implement interwindow drag and drop

services/ui/ws/window_manager_access_policy.cc

Index: services/ui/ws/window_manager_access_policy.cc
diff --git a/services/ui/ws/window_manager_access_policy.cc b/services/ui/ws/window_manager_access_policy.cc
index c815de54cb40ba3b4f22fa18849ead768e967e3e..21e73795314868244bd78f63b769b119aa2f6c4a 100644
--- a/services/ui/ws/window_manager_access_policy.cc
+++ b/services/ui/ws/window_manager_access_policy.cc
@@ -138,6 +138,11 @@ bool WindowManagerAccessPolicy::CanSetHitTestMask(
          delegate_->HasRootForAccessPolicy(window);
 }
 
+bool WindowManagerAccessPolicy::CanSetAcceptDrops(
+    const ServerWindow* window) const {
+  return true;
+}
+
 bool WindowManagerAccessPolicy::CanSetAcceptEvents(
     const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||
@@ -150,6 +155,12 @@ bool WindowManagerAccessPolicy::CanSetCursorProperties(
          delegate_->HasRootForAccessPolicy(window);
 }
 
+bool WindowManagerAccessPolicy::CanInitiateDragLoop(
+    const ServerWindow* window) const {
+  return WasCreatedByThisClient(window) ||
+         delegate_->HasRootForAccessPolicy(window);
+}
+
 bool WindowManagerAccessPolicy::CanInitiateMoveLoop(
     const ServerWindow* window) const {
   return false;