[mojo] Change mojo::ConvertTo presubmit check from error to warning

Bug: 621383
Change-Id: Ie2da4387a8352073c6d2221274b39034c8ee285c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616148
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660665}

PRESUBMIT.py

@@ -775,7 +775,7 @@ _BANNED_CPP_FUNCTIONS = (
         'StringTraits if you would like to convert between custom types and',
         'the wire format of mojom types.'
       ),
-      True,
+      False,
       (
         r'^third_party/blink/.*\.(cc|h)$',
         r'^content/renderer/.*\.(cc|h)$',

PRESUBMIT_test.py

@@ -1938,14 +1938,16 @@ class BannedFunctionCheckTest(unittest.TestCase):
                ['mojo::ConvertTo<>']),
     ]
 
-    errors = PRESUBMIT._CheckNoBannedFunctions(input_api, MockOutputApi())
-    self.assertEqual(1, len(errors))
-    self.assertTrue('some/cpp/problematic/file.cc' in errors[0].message)
-    self.assertTrue('some/cpp/problematic/file2.cc' in errors[0].message)
-    self.assertTrue('some/cpp/ok/file.cc' not in errors[0].message)
-    self.assertTrue('some/cpp/ok/file2.cc' not in errors[0].message)
-    self.assertTrue('third_party/blink/ok/file3.cc' not in errors[0].message)
-    self.assertTrue('content/renderer/ok/file3.cc' not in errors[0].message)
+    results = PRESUBMIT._CheckNoBannedFunctions(input_api, MockOutputApi())
+
+    # warnings are results[0], errors are results[1]
+    self.assertEqual(2, len(results))
+    self.assertTrue('some/cpp/problematic/file.cc' in results[1].message)
+    self.assertTrue('some/cpp/problematic/file2.cc' in results[0].message)
+    self.assertTrue('some/cpp/ok/file.cc' not in results[1].message)
+    self.assertTrue('some/cpp/ok/file2.cc' not in results[1].message)
+    self.assertTrue('third_party/blink/ok/file3.cc' not in results[0].message)
+    self.assertTrue('content/renderer/ok/file3.cc' not in results[0].message)
 
 
 class NoProductionCodeUsingTestOnlyFunctionsTest(unittest.TestCase):