Add support for multiple roots to the client lib.

mojo/services/public/cpp/view_manager/lib/view_manager_private.h

Index: mojo/services/public/cpp/view_manager/lib/view_manager_private.h
diff --git a/mojo/services/public/cpp/view_manager/lib/view_manager_private.h b/mojo/services/public/cpp/view_manager/lib/view_manager_private.h
index 1c091ed56c095d71928f98b77599ac5f8318d500..ff09c158396144f1a7bf645541fbeed6c8d4975e 100644
--- a/mojo/services/public/cpp/view_manager/lib/view_manager_private.h
+++ b/mojo/services/public/cpp/view_manager/lib/view_manager_private.h
@@ -19,10 +19,6 @@ class ViewManagerPrivate {
   explicit ViewManagerPrivate(ViewManager* manager);
   ~ViewManagerPrivate();
 
-  void NotifyReady() {
-    manager_->ready_callback_.Run(manager_);
-  }
-
   ViewManagerSynchronizer* synchronizer() {
     return manager_->synchronizer_;
   }
@@ -30,7 +26,8 @@ class ViewManagerPrivate {
     manager_->synchronizer_ = synchronizer;
   }
 
-  void set_root(ViewTreeNode* root) { manager_->tree_ = root; }
+  void AddRoot(ViewTreeNode* root);
+  void RemoveRoot(ViewTreeNode* root);
 
   void AddNode(TransportNodeId node_id, ViewTreeNode* node);
   void RemoveNode(TransportNodeId node_id);