Introdcue AshWindowTreeHost and move ash/chrome specific code in WTH to ash.

ash/wm/maximize_mode/maximize_mode_window_manager.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 "ash/wm/maximize_mode/maximize_mode_window_manager.h"
 
 #include "ash/root_window_controller.h"
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/wm/maximize_mode/maximize_mode_window_state.h"
@@ skipping 193 matching lines @@
   if (backdrops_hidden_)
     return;
   // Inform the WorkspaceLayoutManager that we want to show a backdrop behind
   // the topmost window of its container.
   Shell::RootWindowControllerList controllers =
       Shell::GetAllRootWindowControllers();
   for (Shell::RootWindowControllerList::iterator iter = controllers.begin();
        iter != controllers.end(); ++iter) {
     RootWindowController* controller = *iter;
     aura::Window* container = Shell::GetContainer(
-        controller->root_window(), kShellWindowId_DefaultContainer);
+        controller->GetRootWindow(), kShellWindowId_DefaultContainer);
     controller->workspace_controller()->SetMaximizeBackdropDelegate(
         scoped_ptr<WorkspaceLayoutManagerDelegate>(
             enable ? new WorkspaceBackdropDelegate(container) : NULL));
   }
 }
 
 }  // namespace ash