dbus: Use more OnceCallbacks in ObjectProxy

Convert WaitForServiceToBeAvailableCallback and OnConnectedCallback to
OnceCallback.

Remove unused MockObjectProxy::WaitForServiceToBeAvailable().
Add MockObjectProxy::DoConnectToSignal() to workaround Gmock's
limitation around move-only arguments.
https://github.com/google/googletest/blob/master/googlemock/docs/CookBook.md#mocking-methods-that-use-move-only-types

BUG=740015
TEST=try

Change-Id: Ide16b01e57eac20f231d729ad570547376a7874d
Reviewed-on: https://chromium-review.googlesource.com/656480
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501180}

CrOS-Libchrome-Original-Commit: 0fc9c713bab87120fa0c78eafa876506333ee235
diff --git a/dbus/mock_object_proxy.cc b/dbus/mock_object_proxy.cc
index 616fb02..1ba754f 100644
--- a/dbus/mock_object_proxy.cc
+++ b/dbus/mock_object_proxy.cc
@@ -30,4 +30,13 @@
                                 &error_callback);
 }
 
+void MockObjectProxy::ConnectToSignal(
+    const std::string& interface_name,
+    const std::string& signal_name,
+    SignalCallback signal_callback,
+    OnConnectedCallback on_connected_callback) {
+  DoConnectToSignal(interface_name, signal_name, signal_callback,
+                    &on_connected_callback);
+}
+
 }  // namespace dbus