Stop showing "Presubmit errors, can't continue (use --no_presubmit to bypass)" when running gcl presubmit!

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/119436

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@18078 0039d316-1c4b-4281-b951-d872f2087c98

gcl.py

@@ -1040,7 +1040,7 @@ def DoPresubmitChecks(change_info, committing, may_prompt):
                                                input_stream=sys.stdin,
                                                default_presubmit=root_presubmit,
                                                may_prompt=may_prompt)
-  if not result:
+  if not result and may_prompt:
     print "\nPresubmit errors, can't continue (use --no_presubmit to bypass)"
   return result