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

ash/shell_delegate.h

Index: ash/shell_delegate.h
diff --git a/ash/shell_delegate.h b/ash/shell_delegate.h
index 8aba55298845be1bba4d03e2ef305548b35633e2..f5c62c3b979303e4a6f14ef6db2ad6760c1d0666 100644
--- a/ash/shell_delegate.h
+++ b/ash/shell_delegate.h
@@ -138,6 +138,9 @@ class ASH_EXPORT ShellDelegate {
 
   // Toggles the status of touchpad between enabled and disabled.
   virtual void ToggleTouchpad() {}
+
+  // Suspends all WebContents-associated media sessions to stop managed players.
+  virtual void SuspendMediaSessions() {}
 };
 
 }  // namespace ash