Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/wm/overview/window_overview.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/wm/overview/window_overview.h"
 
 #include <algorithm>
 
 #include "ash/metrics/user_metrics_recorder.h"
 #include "ash/screen_util.h"
@@ skipping 127 matching lines @@
     cursor_client_->ShowCursor();
     // TODO(flackr): Only prevent cursor changes for windows in the overview.
     // This will be easier to do without exposing the overview mode code if the
     // cursor changes are moved to ToplevelWindowEventHandler::HandleMouseMoved
     // as suggested there.
     cursor_client_->LockCursor();
   }
   ash::Shell::GetInstance()->PrependPreTargetHandler(this);
   Shell* shell = Shell::GetInstance();
   shell->metrics()->RecordUserMetricsAction(UMA_WINDOW_OVERVIEW);
+  shell->OnOverviewModeStarted();
   HideAndTrackNonOverviewWindows();
 }
 
 WindowOverview::~WindowOverview() {
   const aura::WindowTracker::Windows hidden_windows = hidden_windows_.windows();
   for (aura::WindowTracker::Windows::const_iterator iter =
        hidden_windows.begin(); iter != hidden_windows.end(); ++iter) {
     ui::ScopedLayerAnimationSettings settings(
         (*iter)->layer()->GetAnimator());
     settings.SetTransitionDuration(base::TimeDelta::FromMilliseconds(
         ScopedTransformOverviewWindow::kTransitionMilliseconds));
     settings.SetPreemptionStrategy(
         ui::LayerAnimator::IMMEDIATELY_ANIMATE_TO_NEW_TARGET);
     (*iter)->Show();
     (*iter)->layer()->SetOpacity(1);
   }
   if (cursor_client_)
     cursor_client_->UnlockCursor();
-  ash::Shell::GetInstance()->RemovePreTargetHandler(this);
+  Shell* shell = Shell::GetInstance();
+  shell->RemovePreTargetHandler(this);
   UMA_HISTOGRAM_MEDIUM_TIMES(
       "Ash.WindowSelector.TimeInOverview",
       base::Time::Now() - overview_start_time_);
+  shell->OnOverviewModeEnded();

[sky, 2014/03/06 01:03:18]

How about moving this to its own patch?

[Mr4D (OOO till 08-26), 2014/03/06 17:17:16]

It is already. Please look at the latest rev of the patch from yesterday
morning. :)

 }
 
 void WindowOverview::SetSelection(size_t index) {
   gfx::Rect target_bounds(GetSelectionBounds(index));
 
   if (selection_widget_) {
     // If the selection widget is already active, determine the animation to
     // use to animate the widget to the new bounds.
     int change = static_cast<int>(index) - static_cast<int>(selection_index_);
     int windows = static_cast<int>(windows_->size());
@@ skipping 267 matching lines @@
 }
 
 gfx::Rect WindowOverview::GetSelectionBounds(size_t index) {
   gfx::Rect bounds((*windows_)[index]->bounds());
   bounds.Inset(-kWindowOverviewSelectionPadding,
                -kWindowOverviewSelectionPadding);
   return bounds;
 }
 
 }  // namespace ash