Makes it so a node can only the root of one connection at a time

mojo/services/view_manager/root_node_manager.h

Index: mojo/services/view_manager/root_node_manager.h
diff --git a/mojo/services/view_manager/root_node_manager.h b/mojo/services/view_manager/root_node_manager.h
index 62932abf2e607b30f371fcfc5f16ed62b06f8ffe..0b20fdc3fddc52b15616451634bf23a17aa1bfe0 100644
--- a/mojo/services/view_manager/root_node_manager.h
+++ b/mojo/services/view_manager/root_node_manager.h
@@ -69,6 +69,10 @@ class MOJO_VIEW_MANAGER_EXPORT RootNodeManager
       return message_ids_.count(connection_id) > 0;
     }
 
+    // Sends OnServerChangeIdAdvanced() to all connections that have not yet
+    // been messaged.
+    void SendServerChangeIdAdvanced();
+
    private:
     RootNodeManager* root_;
     const ConnectionSpecificId connection_id_;
@@ -133,6 +137,9 @@ class MOJO_VIEW_MANAGER_EXPORT RootNodeManager
       ConnectionSpecificId creator_id,
       const std::string& url) const;
 
+  // Returns the ViewManagerServiceImpl that has |id| as a root.
+  ViewManagerServiceImpl* GetConnectionWithRoot(const NodeId& id);
+
   void DispatchViewInputEventToWindowManager(const View* view,
                                              const ui::Event* event);
 
@@ -178,6 +185,9 @@ class MOJO_VIEW_MANAGER_EXPORT RootNodeManager
   // Balances a call to PrepareForChange().
   void FinishChange();
 
+  // See description in ScopedChange.
+  void SendServerChangeIdAdvanced();
+
   // Returns true if the specified connection originated the current change.
   bool IsChangeSource(ConnectionSpecificId connection_id) const {
     return current_change_ && current_change_->connection_id() == connection_id;