mus: Use the new drag API to implement tab dragging in chrome.

ui/views/mus/window_manager_connection.cc

 // Copyright 2015 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 "ui/views/mus/window_manager_connection.h"
 
 #include <utility>
 
 #include "base/lazy_instance.h"
 #include "base/threading/thread_local.h"
@@ skipping 82 matching lines @@
 }
 
 void WindowManagerConnection::RemovePointerWatcher(PointerWatcher* watcher) {
   pointer_watchers_.RemoveObserver(watcher);
   if (!HasPointerWatcher()) {
     // Last PointerWatcher removed, stop the event observer.
     client_->SetEventObserver(nullptr);
   }
 }
 
+const std::set<mus::Window*>& WindowManagerConnection::GetRoots() const {
+  return client_->GetRoots();
+}
+
 WindowManagerConnection::WindowManagerConnection(
     shell::Connector* connector,
     const shell::Identity& identity)
     : connector_(connector), identity_(identity) {
   lazy_tls_ptr.Pointer()->Set(this);
   client_.reset(new mus::WindowTreeClient(this, nullptr, nullptr));
   client_->ConnectViaWindowTreeFactory(connector_);
 
   screen_.reset(new ScreenMus(this));
   screen_->Init(connector);
@@ skipping 68 matching lines @@
 void WindowManagerConnection::OnWindowManagerFrameValuesChanged() {
   if (client_)
     NativeWidgetMus::NotifyFrameChanged(client_.get());
 }
 
 gfx::Point WindowManagerConnection::GetCursorScreenPoint() {
   return client_->GetCursorScreenPoint();
 }
 
 }  // namespace views