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

BUG=647395

Review-Url: https://codereview.chromium.org/2338293004
Cr-Commit-Position: refs/heads/master@{#419257}

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

@@ -271,6 +271,8 @@ class W3CExpectationsLineAdder(object):
         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)

third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations_unittest.py

@@ -226,7 +226,6 @@ class UpdateW3CTestExpectationsTest(logtesting.LoggingTestCase):
         self.assertMultiLineEqual(
             value,
             ('# Tests added from W3C auto import bot\n'
-             '\n'
              'crbug.com/123 [ FakePlatform ] fake/file/path.html [ Pass ]\n'))
 
     def test_is_js_test_true(self):