Better error message in cc PRESUBMIT

PRESUBMIT checks for usage of FIXME, so added that to error message.

Review URL: https://codereview.chromium.org/515063004

Cr-Commit-Position: refs/heads/master@{#292431}

cc/PRESUBMIT.py

@@ -141,7 +141,8 @@ def CheckTodos(input_api, output_api):
 
   if errors:
     return [output_api.PresubmitError(
-      'All TODO comments should be of the form TODO(name).',
+      'All TODO comments should be of the form TODO(name). ' +
+      'Use TODO instead of FIX' + 'ME',
       items=errors)]
   return []