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

ash/system/power/tablet_power_button_controller.cc

Index: ash/system/power/tablet_power_button_controller.cc
diff --git a/ash/system/power/tablet_power_button_controller.cc b/ash/system/power/tablet_power_button_controller.cc
index b4ed4f18f6f357c189653782b1c4e18e43888c92..4beeb3e4c26ae88b9ab83dba45320895564a63f0 100644
--- a/ash/system/power/tablet_power_button_controller.cc
+++ b/ash/system/power/tablet_power_button_controller.cc
@@ -221,6 +221,9 @@ void TabletPowerButtonController::SetDisplayForcedOff(bool forced_off) {
   backlights_forced_off_ = forced_off;
   UpdateTouchscreenStatus();
 
+  if (backlights_forced_off_)
+    Shell::Get()->shell_delegate()->SuspendMediaSessions();
+
   // Send an a11y alert.
   Shell::Get()->accessibility_delegate()->TriggerAccessibilityAlert(
       forced_off ? A11Y_ALERT_SCREEN_OFF : A11Y_ALERT_SCREEN_ON);