Adds functionality to update-w3c-test-expectations

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 script to modify TestExpectations lines based layout test failures in try jobs.
 
 This script outputs a list of test expectation lines to add to a 'TestExpectations' file
 by retrieving the try job results for the current CL.
 """
 
@@ skipping 212 matching lines @@
                 line_list.append(str(line))
         return line_list
 
     def write_to_test_expectations(self, host, path, line_list):
         """Writes to TestExpectations.
 
         Writes to the test expectations lines in line_list
         to LayoutTest/TestExpectations. Checks the file for the string
         '# Tests added from W3C auto import bot' and writes expectation
         lines directly under it. If not found, it writes to the end of
-        the file.
+        the file. If the test name is already in LayoutTests/TestExpectations,
+        the line will be skipped.
 
         Args:
             host: A Host object.
-            path: The path to the file LayoutTest/TestExpectations.
+            path: The path to the file LayoutTests/TestExpectations.
             line_list: A list of w3c test expectations lines.
 
         Returns:
             Writes to a file on the filesystem called LayoutTests/TestExpectations.
         """
         comment_line = '# Tests added from W3C auto import bot'
         file_contents = host.filesystem.read_text_file(path)
         w3c_comment_line_index = file_contents.find(comment_line)
         all_lines = ''
         for line in line_list:
+            end_bracket_index = line.split().index(']')
+            test_name = line.split()[end_bracket_index + 1]
+            if test_name in file_contents:
+                continue
             all_lines += str(line) + '\n'
         all_lines = all_lines[:-1]
         if w3c_comment_line_index == -1:
             file_contents += '\n%s\n' % comment_line
             file_contents += all_lines
         else:
             end_of_comment_line = (file_contents[w3c_comment_line_index:].find('\n')) + w3c_comment_line_index
             new_data = file_contents[: end_of_comment_line + 1] + all_lines + file_contents[end_of_comment_line:]
             file_contents = new_data
         host.filesystem.write_text_file(path, file_contents)