Improve presubmit check for BUG line

Don't chicken out on upload already, and ignore 'none' value

BUG=none
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/1237353003

Cr-Commit-Position: refs/heads/master@{#29724}

PRESUBMIT.py

@@ -209,28 +209,32 @@ def _SkipTreeCheck(input_api, output_api):
   return input_api.environ.get('PRESUBMIT_TREE_CHECK') == 'skip'
 
 
-def _CheckChangeLogFlag(input_api, output_api):
+def _CheckChangeLogFlag(input_api, output_api, warn):
   """Checks usage of LOG= flag in the commit message."""
   results = []
-  if input_api.change.BUG and not 'LOG' in input_api.change.tags:
-    results.append(output_api.PresubmitError(
-        'An issue reference (BUG=) requires a change log flag (LOG=). '
+  if (input_api.change.BUG and input_api.change.BUG != 'none' and
+      not 'LOG' in input_api.change.tags):
+    text = ('An issue reference (BUG=) requires a change log flag (LOG=). '
             'Use LOG=Y for including this commit message in the change log. '
-        'Use LOG=N or leave blank otherwise.'))
+            'Use LOG=N or leave blank otherwise.')
+    if warn:
+      results.append(output_api.PresubmitPromptWarning(text))
+    else:
+      results.append(output_api.PresubmitError(text))
   return results
 
 
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
-  results.extend(_CheckChangeLogFlag(input_api, output_api))
+  results.extend(_CheckChangeLogFlag(input_api, output_api, True))
   return results
 
 
 def CheckChangeOnCommit(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
-  results.extend(_CheckChangeLogFlag(input_api, output_api))
+  results.extend(_CheckChangeLogFlag(input_api, output_api, False))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   if not _SkipTreeCheck(input_api, output_api):