Add support for multiple roots to the client lib.

mojo/examples/sample_view_manager_app/sample_view_manager_app.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/at_exit.h"
 #include "base/bind.h"
 #include "base/strings/stringprintf.h"
 #include "mojo/public/cpp/application/application.h"
 #include "mojo/public/cpp/environment/environment.h"
 #include "mojo/public/cpp/system/core.h"
@@ skipping 47 matching lines @@
     view_manager::ViewTreeNode* node1 =
         view_manager::ViewTreeNode::Create(view_manager_.get());
     node1->SetBounds(gfx::Rect(800, 600));
     view_manager::ViewTreeNode* node11 =
         view_manager::ViewTreeNode::Create(view_manager_.get());
     node11->SetBounds(gfx::Rect(800, 600));
 
     view_manager::View* view11 =
         view_manager::View::Create(view_manager_.get());
     node11->SetActiveView(view11);
-    view_manager_->tree()->AddChild(node1);
+    view_manager_->roots().front()->AddChild(node1);
     node1->AddChild(node11);
 
     view11->SetColor(SK_ColorRED);
 
     view11->AddObserver(this);
   }
 
  private:
   virtual void OnViewInputEvent(mojo::view_manager::View* view,
                                 EventPtr event) OVERRIDE {
@@ skipping 15 matching lines @@
 };
 
 }  // namespace examples
 
 // static
 Application* Application::Create() {
   return new examples::SampleApp();
 }
 
 }  // namespace mojo