Fix presubmit check for webrtc/base.

PRESUBMIT.py

 # Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import json
 import os
@@ skipping 504 matching lines @@
   problems = []
 
   for f in input_api.AffectedFiles():
     if os.path.join('webrtc', 'base') in f.LocalPath():
       problems.append('    ' + f.LocalPath())
       continue
     for line_num, line in f.ChangedContents():
       if 'webrtc/base' in line:
         problems.append('    %s: %s' % (f.LocalPath(), line_num))
 
-  return [output_api.PresubmitPromptWarning(
-      'webrtc/base is being moved to webrtc/rtc_base (See '
-      'bugs.webrtc.org/7634). Please refer to webrtc/rtc_base instead in the '
-      'following files:\n' + '\n'.join(problems))]
+  if problems:
+    return [output_api.PresubmitPromptWarning(
+        'webrtc/base is being moved to webrtc/rtc_base (See '
+        'bugs.webrtc.org/7634). Please refer to webrtc/rtc_base instead in the '
+        'following files:\n' + '\n'.join(problems))]
+  return []
 
 
 def _CommonChecks(input_api, output_api):
   """Checks common to both upload and commit."""
   results = []
   # Filter out files that are in objc or ios dirs from being cpplint-ed since
   # they do not follow C++ lint rules.
   black_list = input_api.DEFAULT_BLACK_LIST + (
     r".*\bobjc[\\\/].*",
     r".*objc\.[hcm]+$",
@@ skipping 121 matching lines @@
   results = []
   source_file_filter = lambda x: input_api.FilterSourceFile(
       x, white_list=(r'.+\.proto$',))
   for f in input_api.AffectedSourceFiles(source_file_filter):
     file_path = f.LocalPath()
     with open(file_path) as f:
       lines = f.readlines()
       if lines[-1] != '\n' or lines[-2] == '\n':
         results.append(output_api.PresubmitError(error_msg.format(file_path)))
   return results