update_engine: Refresh device policy when it is updated.

The session manager emits a signal whenever the device policy is
updated. This patch makes update_engine listen for that signal and
reload the device policy.

BUG=chromium:423037
TEST=deployed update_engine and checked the policy is refreshed.

Change-Id: I71dd7047e53d49f3402e1f9f4a67ec8cbd3739d3
Reviewed-on: https://chromium-review.googlesource.com/235884
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
diff --git a/mock_dbus_wrapper.h b/mock_dbus_wrapper.h
index 9c1f7c0..5d630a6 100644
--- a/mock_dbus_wrapper.h
+++ b/mock_dbus_wrapper.h
@@ -41,6 +41,10 @@
                                        const char* in2,
                                        const char* in3));
 
+  MOCK_METHOD3(ProxyAddSignal_1, void(DBusGProxy* proxy,
+                                      const char* signal_name,
+                                      GType type1));
+
   MOCK_METHOD4(ProxyAddSignal_2, void(DBusGProxy* proxy,
                                       const char* signal_name,
                                       GType type1,