Add signal handler for low disk notification

chromeos/dbus/fake_cryptohome_client.h

Index: chromeos/dbus/fake_cryptohome_client.h
diff --git a/chromeos/dbus/fake_cryptohome_client.h b/chromeos/dbus/fake_cryptohome_client.h
index 9fb1f493e9a693440d328df663bc71c85dc02997..b52751657d2f41a41b9c1d9c02232a7387be175e 100644
--- a/chromeos/dbus/fake_cryptohome_client.h
+++ b/chromeos/dbus/fake_cryptohome_client.h
@@ -25,6 +25,7 @@ class CHROMEOS_EXPORT FakeCryptohomeClient : public CryptohomeClient {
       const AsyncCallStatusHandler& handler,
       const AsyncCallStatusWithDataHandler& data_handler) override;
   void ResetAsyncCallStatusHandlers() override;
+  void SetLowDiskSpaceHandler(const LowDiskSpaceHandler& handler) override;
   void WaitForServiceToBeAvailable(
       const WaitForServiceToBeAvailableCallback& callback) override;
   void IsMounted(const BoolDBusMethodCallback& callback) override;