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..2daed3b9e69e89c1429d354ed35b188db6fb6e4a 100644
--- a/services/ui/ws/default_access_policy.cc
+++ b/services/ui/ws/default_access_policy.cc
@@ -149,6 +149,10 @@ bool DefaultAccessPolicy::CanSetHitTestMask(const ServerWindow* window) const {
          delegate_->HasRootForAccessPolicy(window);
 }
 
+bool DefaultAccessPolicy::CanSetAcceptDrops(const ServerWindow* window) const {
+  return delegate_->HasRootForAccessPolicy(window);

[sky, 2016/09/08 23:37:20]

This only accepts drops on the root. You also want
WasCreatedByThisClient(window) here, otherwise windows created by this client
can't accept drops. I think you'll also want to forbid the drop if there is
another client embedded in the window. In other words I think you want the same
thing as is in DefaultAccessPolicy::CanDescendIntoWindowForWindowTree(). I think
you want the same in the WindowManagerAccessPolicy.

+}
+
 bool DefaultAccessPolicy::CanSetAcceptEvents(const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||
          delegate_->HasRootForAccessPolicy(window);