AU: replace sleep/usleep with g_usleep

Also deploying 'using base::TimeDelta' where it is useful.

BUG=chromium-os:33541
TEST=Passes unit tests; update runs on x86-alex

Change-Id: I9478d46d0796b13789878393e3a4086564124d33
Reviewed-on: https://gerrit.chromium.org/gerrit/40904
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/update_engine_client.cc b/update_engine_client.cc
index f8396c9..9d5767c 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -50,7 +50,7 @@
     if (i > 0) {
       LOG(INFO) << "Retrying to get dbus proxy. Try "
                 << (i + 1) << "/" << kTries;
-      sleep(kRetrySeconds);
+      g_usleep(kRetrySeconds * G_USEC_PER_SEC);
     }
     proxy = dbus_g_proxy_new_for_name_owner(bus,
                                             kUpdateEngineServiceName,