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..3ee5ad633c08138ccb3180186905bf0d9e1f4ae9 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;
@@ -228,6 +229,7 @@ class CHROMEOS_EXPORT FakeCryptohomeClient : public CryptohomeClient {
   int async_call_id_;
   AsyncCallStatusHandler async_call_status_handler_;
   AsyncCallStatusWithDataHandler async_call_status_data_handler_;
+  LowDiskSpaceHandler low_disk_space_handler_;
   bool unmount_result_;
   std::vector<uint8_t> system_salt_;