Reify view ownership as a message pipe.

mojo/services/ui/views/interfaces/view_trees.mojom

Index: mojo/services/ui/views/interfaces/view_trees.mojom
diff --git a/mojo/services/ui/views/interfaces/view_trees.mojom b/mojo/services/ui/views/interfaces/view_trees.mojom
index ff7a426ec55a06bae9dc830300af807503e7942e..cb545aba8f9df54fcd57eb2a155b3223c3474172 100644
--- a/mojo/services/ui/views/interfaces/view_trees.mojom
+++ b/mojo/services/ui/views/interfaces/view_trees.mojom
@@ -62,6 +62,9 @@ interface ViewTree {
 // ViewManager.  To unregister the view tree, close its view tree
 // and/or view tree host message pipes.
 interface ViewTreeHost {
+  // Gets the view tree's token.
+  GetToken() => (ViewTreeToken token);
+
   // Gets a service provider to access services which are associated with
   // the view tree such as input, accessibility and editing capabilities.
   // The view tree service provider is private to the view tree and should
@@ -81,23 +84,31 @@ interface ViewTreeHost {
   // is set so that callbacks related to the root can be clearly distinguished
   // across these changes.
   //
-  // If |root_view_token| refers to a view which is already unavailable
+  // If |root_view_owner| refers to a view which is already unavailable
   // then the call proceeds as if it succeeded but an OnChildUnavailable()
   // message will be sent.
   //
-  // If |root_view_token| refers to a view which already has a parent or is
+  // If |root_view_owner| refers to a view which already has a parent or is
   // the root of a view tree then an OnChildUnavailable() or OnRootUnavailable()
   // message will be sent to its old parent or root and the the view will be
   // used as the root of the new view tree as usual.  This special case also
   // applies when the specified view is already the root of this view tree, in
   // which case the behavior is similar to the view having been transferred to
   // some other view tree and then back again.
-  SetRoot(uint32 root_key, mojo.ui.ViewToken root_view_token);
+  //
+  // It is an error to call this function if the root has already been set;
+  // the connection will be closed.
+  SetRoot(uint32 root_key, mojo.ui.ViewOwner root_view_owner);
 
   // Removes the root of the view tree.
   //
-  // Does nothing if the view tree currently does not have a root.
-  ResetRoot();
+  // If |transferred_view_owner| is not null, associates it with the previously
+  // configured view or closes the |transferred_view_owner| message pipe
+  // if there was none.
+  //
+  // It is an error to call this function if the root was not previously set;
+  // the connection will be closed.
+  ResetRoot(mojo.ui.ViewOwner&? transferred_view_owner);
 
   // Sets the layout parameters of the root of the view tree and retrieves
   // its layout information.