Use g_strdup() instead of strdup().

The strings are freed in dbus-glib with g_free(), hence we should use
g_strdup().

TEST=confirmed that update_engine worked as before
BUG=chromium-os:8197

Change-Id: Idf07d1ded34b7f55ac78d16b74fabb5701038068

Review URL: http://codereview.chromium.org/4097004
diff --git a/dbus_service.cc b/dbus_service.cc
index a27fd18..5a1a105 100644
--- a/dbus_service.cc
+++ b/dbus_service.cc
@@ -90,8 +90,8 @@
                                            &new_version_str,
                                            new_size));
 
-  *current_operation = strdup(current_op.c_str());
-  *new_version = strdup(new_version_str.c_str());
+  *current_operation = g_strdup(current_op.c_str());
+  *new_version = g_strdup(new_version_str.c_str());
   if (!(*current_operation && *new_version)) {
     *error = NULL;
     return FALSE;
@@ -104,7 +104,7 @@
                                          GError **error) {
   string track_str =
       chromeos_update_engine::OmahaRequestDeviceParams::GetDeviceTrack();
-  *track = strdup(track_str.c_str());
+  *track = g_strdup(track_str.c_str());
   return TRUE;
 }