chromeos: Add support for dimming the screen.

ash/wm/screen_dimmer.h

Index: ash/wm/screen_dimmer.h
diff --git a/ash/wm/screen_dimmer.h b/ash/wm/screen_dimmer.h
new file mode 100644
index 0000000000000000000000000000000000000000..2415b5807d764cbc54651615256c5ad3916d0676
--- /dev/null
+++ b/ash/wm/screen_dimmer.h
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+// Copyright (c) 2012 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.
+
+#ifndef ASH_WM_SCREEN_DIMMER_H_
+#define ASH_WM_SCREEN_DIMMER_H_
+#pragma once
+
+#include "ash/ash_export.h"
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+#include "ui/aura/root_window_observer.h"
+
+namespace ui {
+class Layer;
+}
+
+namespace ash {
+namespace internal {
+
+// ScreenDimmer displays a partially-opaque layer above everything else to
+// darken the screen.  It shouldn't be used for long-term brightness adjustments
+// due to performance considerations -- it's only intended for cases where we
+// want to briefly dim the screen (e.g. to indicate to the user that we're about
+// to suspend a machine that lacks an internal backlight that can be adjusted).
+class ASH_EXPORT ScreenDimmer : public aura::RootWindowObserver {
+ public:
+  class TestApi {
+   public:
+    explicit TestApi(ScreenDimmer* dimmer) : dimmer_(dimmer) {}
+
+    ui::Layer* layer() { return dimmer_->dimming_layer_.get(); }
+
+   private:
+    ScreenDimmer* dimmer_;  // not owned
+
+    DISALLOW_COPY_AND_ASSIGN(TestApi);
+  };
+
+  ScreenDimmer();
+  virtual ~ScreenDimmer();
+
+  // Dim or undim the screen.
+  void SetDimming(bool should_dim);
+
+  // aura::RootWindowObserver overrides:
+  virtual void OnRootWindowResized(const aura::RootWindow* root,
+                                   const gfx::Size& old_size) OVERRIDE;
+
+ private:
+  friend class TestApi;
+
+  // Partially-opaque layer that's stacked above all of the root window's
+  // children and used to dim the screen.  NULL until the first time we dim.
+  scoped_ptr<ui::Layer> dimming_layer_;
+
+  // Are we currently dimming the screen?
+  bool currently_dimming_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScreenDimmer);
+};
+
+}  // namespace internal
+}  // namespace ash
+
+#endif  // ASH_WM_SCREEN_DIMMER_H_