In update_w3c_test_expectations, do nothing when there are no new lines to add.

third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations.py

 # Copyright 2016 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.
 
 """A class for updating layout test expectations when updating w3c tests.
 
 Specifically, this class fetches results from try bots for the current CL, and:
   1. Downloads new baseline files for any tests that can be rebaselined.
   2. Updates the generic TestExpectations file for any other failing tests.
 
@@ skipping 253 matching lines @@
         Args:
             line_list: A list of lines to add to the TestExpectations file.
         """
         _log.debug('Lines to write to TestExpectations: %r', line_list)
         port = self.host.port_factory.get()
         expectations_file_path = port.path_to_generic_test_expectations_file()
         marker_comment = '# Tests added from W3C auto import bot'
         file_contents = self.host.filesystem.read_text_file(expectations_file_path)
         marker_comment_index = file_contents.find(marker_comment)
         line_list = [line for line in line_list if self._test_name_from_expectation_string(line) not in file_contents]
+        if not line_list:
+            return
         if marker_comment_index == -1:
             file_contents += '\n%s\n' % marker_comment
             file_contents += '\n'.join(line_list)
         else:
             end_of_marker_line = (file_contents[marker_comment_index:].find('\n')) + marker_comment_index
             file_contents = file_contents[:end_of_marker_line + 1] + '\n'.join(line_list) + file_contents[end_of_marker_line:]
         self.host.filesystem.write_text_file(expectations_file_path, file_contents)
 
     @staticmethod
     def _test_name_from_expectation_string(expectation_string):
@@ skipping 66 matching lines @@
 
         Args:
             test_path: A file path relative to the layout tests directory.
                 This might correspond to a deleted file or a non-test.
         """
         absolute_path = self.host.filesystem.join(self.finder.layout_tests_dir(), test_path)
         test_parser = TestParser(absolute_path, self.host)
         if not test_parser.test_doc:
             return False
         return test_parser.is_jstest()