Allow Alt-Tab to move the focus to docked windows.

ash/switchable_windows.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/switchable_windows.h"
 
 #include "ash/shell_window_ids.h"
 #include "ui/aura/window.h"
 
 namespace ash {
 
 const int kSwitchableWindowContainerIds[] = {
     kShellWindowId_DefaultContainer,
     kShellWindowId_AlwaysOnTopContainer,
+    kShellWindowId_DockedContainer,
     kShellWindowId_PanelContainer,
     kShellWindowId_AppListContainer};
 
 const size_t kSwitchableWindowContainerIdsLength =
     arraysize(kSwitchableWindowContainerIds);
 
 bool IsSwitchableContainer(const aura::Window* window) {
   if (!window)
     return false;
   for (size_t i = 0; i < kSwitchableWindowContainerIdsLength; ++i) {
     if (window->id() == kSwitchableWindowContainerIds[i])
       return true;
   }
   return false;
 }
 
 }  // namespace ash