[eslint] Don't treat eslint warnings as PRESUBMIT errors

catapult_build/js_checks.py

 # Copyright (c) 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import re
 
 import eslint
 from py_vulcanize import strip_js_comments
 
 from catapult_build import parse_html
@@ skipping 66 matching lines @@
     for f in affected_js_files:
       contents = list(f.NewContents())
       error_lines += CheckStrictMode(
           '\n'.join(contents),
           is_html_file=f.LocalPath().endswith('.html'))
 
       for i, line in enumerate(contents, start=1):
         error_lines += filter(None, [self.ConstCheck(i, line)])
 
     if affected_js_files:
-      eslint_output = eslint.RunEslintOnFiles(
-          [f.AbsoluteLocalPath() for f in affected_js_files]).rstrip()
+      success, eslint_output = eslint.RunEslint(
+          [f.AbsoluteLocalPath() for f in affected_js_files])
 
-      if eslint_output:
+      if not success:
         error_lines.append('\neslint found lint errors:')
         error_lines.append(eslint_output)
 
     if error_lines:
       error_lines.insert(0, 'Found JavaScript style violations:')
       results.append(
           _MakeErrorOrWarning(self.output_api, '\n'.join(error_lines)))
 
     return results
 
@@ skipping 39 matching lines @@
 
 def RunChecks(input_api, output_api, excluded_paths=None):
 
   def ShouldCheck(affected_file):
     if not excluded_paths:
       return True
     path = affected_file.LocalPath()
     return not any(re.match(pattern, path) for pattern in excluded_paths)
 
   return JSChecker(input_api, output_api, file_filter=ShouldCheck).RunChecks()