Prioritize ARC instance only when it is needed

chrome/browser/chromeos/arc/boot_phase_monitor/arc_instance_throttle.h

+// Copyright 2017 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 CHROME_BROWSER_CHROMEOS_ARC_BOOT_PHASE_MONITOR_ARC_INSTANCE_THROTTLE_H_
+#define CHROME_BROWSER_CHROMEOS_ARC_BOOT_PHASE_MONITOR_ARC_INSTANCE_THROTTLE_H_
+
+#include "ash/common/wm_activation_observer.h"
+#include "base/macros.h"
+
+namespace arc {
+
+// A class that watches window activations and prioritizes the ARC instance when
+// one of ARC windows is activated. The class also unprioritizes the instance
+// when non-ARC window such as Chrome is activated.
+class ArcInstanceThrottle : public ash::WmActivationObserver {
+ public:
+  ArcInstanceThrottle();
+  ~ArcInstanceThrottle() override;
+
+  // ash::WmActivationObserver overrides:
+  void OnWindowActivated(ash::WmWindow* gained_active,
+                         ash::WmWindow* lost_active) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ArcInstanceThrottle);
+};
+
+}  // namespace arc
+
+#endif  // CHROME_BROWSER_CHROMEOS_ARC_BOOT_PHASE_MONITOR_ARC_INSTANCE_THROTTLE_H_