webkitpy: Small changes to merging script

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/merge_results.py

 # Copyright 2017 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.
 
 """Classes for merging layout tests results directories together.
 
 This is split into three parts:
 
  * Generic code to merge JSON data together.
  * Generic code to merge directories together.
@@ skipping 412 matching lines @@
 
         self.filesystem = filesystem or FileSystem()
 
         # Default to just checking the file contents matches.
         self.add_helper(lambda *args: True, MergeFilesMatchingContents(self.filesystem))
         # Copy the file it it's the only one.
         self.add_helper(lambda _, to_merge: len(to_merge) == 1, MergeFilesOne(self.filesystem))
 
     def merge(self, output_dir, to_merge_dirs):
         output_dir = self.filesystem.realpath(self.filesystem.abspath(output_dir))
-        if self.filesystem.exists(output_dir):
-            raise OSError("Output directory %s exists." % output_dir)
 
         merge_dirs = []
         # Normalize the given directory values.
         for base_dir in to_merge_dirs:
             merge_dirs.append(self.filesystem.realpath(self.filesystem.abspath(base_dir)))
         merge_dirs.sort()
 
         _log.debug("Merging following paths:")
         _log.debug(DeferredPrettyPrint(merge_dirs))
 
@@ skipping 156 matching lines @@
 
         self.add_helper(
             FilenameMatch('failing_results.json'),
             results_json_file_merger)
         self.add_helper(
             FilenameMatch('full_results.json'),
             results_json_file_merger)
         self.add_helper(
             FilenameMatch('output.json'),
             results_json_file_merger)