Actually make use of the may_prompt flag.

tests/presubmit_unittest.py

Index: tests/presubmit_unittest.py
===================================================================
--- tests/presubmit_unittest.py	(revision 18139)
+++ tests/presubmit_unittest.py	(working copy)
@@ -380,7 +380,7 @@
     output = StringIO.StringIO()
     input = StringIO.StringIO('n\n')  # say no to the warning
     self.failIf(presubmit.DoPresubmitChecks(ci, False, True, output, input,
-                                            None, False))
+                                            None, True))
     self.assertEqual(output.getvalue().count('??'), 2)
 
     output = StringIO.StringIO()
@@ -391,7 +391,7 @@
                                                 output,
                                                 input,
                                                 None,
-                                                False))
+                                                True))
     self.assertEquals(output.getvalue().count('??'), 2)
 
   def testDoPresubmitChecksNoWarningPromptIfErrors(self):