cros: Suspend media sessions with display off trigger by tablet power button

ash/system/power/tablet_power_button_controller.cc

 // Copyright 2016 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/system/power/tablet_power_button_controller.h"
 
 #include "ash/accessibility_delegate.h"
 #include "ash/session/session_controller.h"
 #include "ash/shell.h"
 #include "ash/shell_delegate.h"
@@ skipping 204 matching lines @@
   if (backlights_forced_off_ == forced_off)
     return;
 
   // Set the display and keyboard backlights (if present) to |forced_off|.
   chromeos::DBusThreadManager::Get()
       ->GetPowerManagerClient()
       ->SetBacklightsForcedOff(forced_off);
   backlights_forced_off_ = forced_off;
   UpdateTouchscreenStatus();
 
+  if (backlights_forced_off_) {
+    ShellDelegate* delegate = Shell::Get()->shell_delegate();
+    delegate->SuspendMediaSessions();

[Daniel Erat, 2017/05/22 23:38:34]

nit: i'd put this on a single line since you only use the delegate once:

  Shell::Get()->shell_delegate()->SuspendMediaSessions();

[Qiang(Joe) Xu, 2017/05/23 17:54:46]

Done.

+  }
+
   // Send an a11y alert.
   Shell::Get()->accessibility_delegate()->TriggerAccessibilityAlert(
       forced_off ? A11Y_ALERT_SCREEN_OFF : A11Y_ALERT_SCREEN_ON);
 }
 
 void TabletPowerButtonController::GetInitialBacklightsForcedOff() {
   chromeos::DBusThreadManager::Get()
       ->GetPowerManagerClient()
       ->GetBacklightsForcedOff(base::Bind(
           &TabletPowerButtonController::OnGotInitialBacklightsForcedOff,
@@ skipping 29 matching lines @@
   SessionController* session_controller = Shell::Get()->session_controller();
   if (session_controller->ShouldLockScreenAutomatically() &&
       session_controller->CanLockScreen() &&
       !session_controller->IsUserSessionBlocked() &&
       !controller_->LockRequested()) {
     session_controller->LockScreen();
   }
 }
 
 }  // namespace ash