Implement "pinned" mode in ash.

ash/common/wm/window_state.cc

 // Copyright 2013 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/common/wm/window_state.h"
 
 #include <utility>
 
 #include "ash/common/wm/default_state.h"
 #include "ash/common/wm/window_positioning_utils.h"
@@ skipping 168 matching lines @@
 }
 
 void WindowState::Restore() {
   if (!IsNormalStateType()) {
     const WMEvent event(WM_EVENT_NORMAL);
     OnWMEvent(&event);
   }
 }
 
 void WindowState::DisableAlwaysOnTop(WmWindow* window_on_top) {
-  DCHECK(window_on_top);
   if (GetAlwaysOnTop()) {
     // |window_| is hidden first to avoid canceling fullscreen mode when it is
     // no longer always on top and gets added to default container. This avoids
     // sending redundant OnFullscreenStateChanged to the layout manager. The
     // |window_| visibility is restored after it no longer obscures the
     // |window_on_top|.
     bool visible = window_->IsVisible();
     if (visible)
       window_->Hide();
     window_->SetAlwaysOnTop(false);
     // Technically it is possible that a |window_| could make itself
     // always_on_top really quickly. This is probably not a realistic case but
     // check if the two windows are in the same container just in case.
-    if (window_on_top->GetParent() == window_->GetParent())
+    if (window_on_top && window_on_top->GetParent() == window_->GetParent())
       window_->GetParent()->StackChildAbove(window_on_top, window_);
     if (visible)
       window_->Show();
     cached_always_on_top_ = true;
   }
 }
 
 void WindowState::RestoreAlwaysOnTop() {
   if (delegate() && delegate()->RestoreAlwaysOnTop(this))
     return;
@@ skipping 184 matching lines @@
   if (!window_->GetTargetVisibility()) {
     SetBoundsConstrained(new_bounds);
     return;
   }
 
   window_->SetBoundsDirectCrossFade(new_bounds);
 }
 
 }  // namespace wm
 }  // namespace ash