Fix broken JSON validator. Needs list and not a string.

BUG=none
TEST=Ran against repo w/ JSON and without JSON changes.

Change-Id: I37590abc7622bbfdfa13f7346e4aa27d848d2164
diff --git a/pre-upload.py b/pre-upload.py
index 7752483..b992489 100644
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -266,7 +266,7 @@
 
 def _run_json_check(project, commit):
   """Checks that all JSON files are syntactically valid."""
-  for f in _filter_files(_get_affected_files(commit), r'.*\.json'):
+  for f in _filter_files(_get_affected_files(commit), [r'.*\.json']):
     try:
       json.load(open(f))
     except Exception, e: