[git_cl] Fix git cl comments json stdout printing

Print "git cl comments" json to stdout when the output file is "-"

Change-Id: I91021317e52019a0d499cccc4f40f062127b300e
Reviewed-on: https://chromium-review.googlesource.com/1228094
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index 1204fed..997d179 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -4714,8 +4714,7 @@
       dct = c.__dict__.copy()
       dct['date'] = dct['date'].strftime('%Y-%m-%d %H:%M:%S.%f')
       return dct
-    with open(options.json_file, 'wb') as f:
-      json.dump(map(pre_serialize, summary), f)
+    write_json(options.json_file, map(pre_serialize, summary))
   return 0