update_engine: Use org.chromium.NetworkProxyService.

Make update_engine call Chrome's new
org.chromium.NetworkProxyService D-Bus service to resolve
network proxies instead of using
org.chromium.LibCrosService. The new service supports
asynchronous replies instead of responding via D-Bus
signals.

BUG=chromium:446115,chromium:703217
TEST=unit tests pass; also added debug logging and verified
     that chrome's proxy settings are used

Change-Id: Iebd268ea3e551c0760416d955828b9d7ebf851fb
Reviewed-on: https://chromium-review.googlesource.com/497491
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/update_manager/real_system_provider.cc b/update_manager/real_system_provider.cc
index fde6bfa..395bf83 100644
--- a/update_manager/real_system_provider.cc
+++ b/update_manager/real_system_provider.cc
@@ -20,9 +20,9 @@
 #include <base/callback.h>
 #include <base/logging.h>
 #include <base/time/time.h>
+#include <libcros/dbus-proxies.h>
 
 #include "update_engine/common/utils.h"
-#include "update_engine/libcros_proxy.h"
 #include "update_engine/update_manager/generic_variables.h"
 #include "update_engine/update_manager/variable.h"
 
@@ -124,9 +124,8 @@
     string* required_platform_version) {
 #if USE_LIBCROS
   brillo::ErrorPtr error;
-  if (!libcros_proxy_->service_interface_proxy()
-           ->GetKioskAppRequiredPlatformVersion(required_platform_version,
-                                                &error)) {
+  if (!libcros_proxy_->GetKioskAppRequiredPlatformVersion(
+          required_platform_version, &error)) {
     LOG(WARNING) << "Failed to get kiosk required platform version";
     required_platform_version->clear();
     return false;