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..d016c94375e4643ad312cdf26c231dcf843ef3d1 100644
--- a/services/ui/ws/window_manager_access_policy.cc
+++ b/services/ui/ws/window_manager_access_policy.cc
@@ -138,6 +138,12 @@ bool WindowManagerAccessPolicy::CanSetHitTestMask(
          delegate_->HasRootForAccessPolicy(window);
 }
 
+bool WindowManagerAccessPolicy::CanSetAcceptDrags(
+    const ServerWindow* window) const {
+  // Only the client associated with a window may set this.

[sky, 2016/09/06 21:11:27]

Are you sure you don't want WasCreatedByThisClient(window)? Otherwise it means
any windows created by the window manager can't accept drags.

+  return false;
+}
+
 bool WindowManagerAccessPolicy::CanSetAcceptEvents(
     const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||