Make sure PresubmitResult can be converted to a json.

R=martiniss@google.com

Change-Id: Ida198bd834e39cc76d483a88a894f8e39d346976
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1681082
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Debrian Figueroa <debrian@google.com>

presubmit_support.py

@@ -306,7 +306,7 @@ class _PresubmitResult(object):
   def json_format(self):
     return {
         'message': self._message,
-        'items': self._items,
+        'items': [str(item) for item in self._items],
         'long_text': self._long_text,
         'fatal': self.fatal
     }