core: Fix differing sequence timestamp precision expectations

Calculations were being done at places accounting on usec precision,
however those are still treated as having msec precision at places. Let's
consolidate for the latter since it requires less changes across the board
and usec precision doesn't buy us anything here.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/541

src/core/startup-notification.c

@@ -37,7 +37,7 @@
  * OpenOffice or whatever seems to stop launching - people
  * might decide they need to launch it again.
  */
-#define STARTUP_TIMEOUT 15000000
+#define STARTUP_TIMEOUT_MS 15000
 
 enum
 {
@@ -425,10 +425,10 @@ collect_timed_out_foreach (void *element,
 
   meta_topic (META_DEBUG_STARTUP,
               "Sequence used %" G_GINT64_FORMAT " ms vs. %d max: %s\n",
-              elapsed, STARTUP_TIMEOUT,
+              elapsed, STARTUP_TIMEOUT_MS,
               meta_startup_sequence_get_id (sequence));
 
-  if (elapsed > STARTUP_TIMEOUT)
+  if (elapsed > STARTUP_TIMEOUT_MS)
     ctod->list = g_slist_prepend (ctod->list, sequence);
 }
 
@@ -440,7 +440,7 @@ startup_sequence_timeout (void *data)
   GSList *l;
 
   ctod.list = NULL;
-  ctod.now = g_get_monotonic_time ();
+  ctod.now = g_get_monotonic_time () / 1000;
   g_slist_foreach (sn->startup_sequences,
                    collect_timed_out_foreach,
                    &ctod);