bluetooth: Forward declare SequencedTaskRunner in BluetoothAdapterChromeOS

device/bluetooth/bluetooth_adapter_chromeos.h

Index: device/bluetooth/bluetooth_adapter_chromeos.h
diff --git a/device/bluetooth/bluetooth_adapter_chromeos.h b/device/bluetooth/bluetooth_adapter_chromeos.h
index 1811287ada8a32296f05095f7623608cacc8443e..13942d8c387bd16ea96207c0bf377b2d2cb2fdd2 100644
--- a/device/bluetooth/bluetooth_adapter_chromeos.h
+++ b/device/bluetooth/bluetooth_adapter_chromeos.h
@@ -12,7 +12,6 @@
 #include <vector>
 
 #include "base/memory/weak_ptr.h"
-#include "base/sequenced_task_runner.h"
 #include "chromeos/dbus/bluetooth_adapter_client.h"
 #include "chromeos/dbus/bluetooth_agent_service_provider.h"
 #include "chromeos/dbus/bluetooth_device_client.h"
@@ -26,6 +25,10 @@
 #include "device/bluetooth/bluetooth_discovery_session.h"
 #include "device/bluetooth/bluetooth_export.h"
 
+namespace base {
+class SequencedTaskRunner;
+}  // namespace base
+
 namespace device {
 class BluetoothSocketThread;
 }  // namespace device