Make generate_perf_json.py work regardless of starting working directory

tools/perf/generate_perf_json.py

 #!/usr/bin/env python
 # 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.
 
 """Script to generate chromium.perf.json and chromium.perf.fyi.json in
 the src/testing/buildbot directory. Maintaining these files by hand is
 too unwieldy.
 """
 
@@ skipping 509 matching lines @@
     'rasterize_and_record_micro_ct',
     'repaint_ct',
     'multipage_skpicture_printer',
     'multipage_skpicture_printer_ct',
     'skpicture_printer',
     'skpicture_printer_ct',
 ]
 
 
 def current_benchmarks(use_whitelist):
-  current_dir = os.path.dirname(__file__)
-  benchmarks_dir = os.path.join(current_dir, 'benchmarks')
+  benchmarks_dir = os.path.join(os.getcwd(), 'benchmarks')
   top_level_dir = os.path.dirname(benchmarks_dir)
 
   all_benchmarks = discover.DiscoverClasses(
       benchmarks_dir, top_level_dir, benchmark_module.Benchmark,
       index_by_class_name=True).values()
   # Remove all blacklisted benchmarks
   for blacklisted in BENCHMARK_NAME_BLACKLIST:
     for benchmark in all_benchmarks:
       if benchmark.Name() == blacklisted:
         all_benchmarks.remove(benchmark)
@@ skipping 100 matching lines @@
         scripts = generate_script_tests(waterfall['name'], name, shard + 1)
         if scripts:
           tests[tester_name] = {
             'scripts': sorted(scripts, key=lambda x: x['name'])
           }
 
   tests['AAAAA1 AUTOGENERATED FILE DO NOT EDIT'] = {}
   tests['AAAAA2 See //tools/perf/generate_perf_json.py to make changes'] = {}
   filename = '%s.json' % waterfall['name']
 
-  current_dir = os.path.dirname(os.path.abspath(__file__))
-  src_dir = os.path.dirname(os.path.dirname(current_dir))
+  src_dir = os.path.dirname(os.path.dirname(os.getcwd()))
 
   with open(os.path.join(src_dir, 'testing', 'buildbot', filename), 'w') as fp:
     json.dump(tests, fp, indent=2, separators=(',', ': '), sort_keys=True)
     fp.write('\n')
 
+def chdir_to_parent_directory():
+  parent_directory = os.path.dirname(os.path.abspath(__file__))
+  os.chdir(parent_directory)
 
 def main():
+  chdir_to_parent_directory()
+
   waterfall = get_waterfall_config()
   waterfall['name'] = 'chromium.perf'
   fyi_waterfall = get_fyi_waterfall_config()
   fyi_waterfall['name'] = 'chromium.perf.fyi'
 
   generate_all_tests(fyi_waterfall)
   generate_all_tests(waterfall)
   return 0
 
 if __name__ == '__main__':
   sys.exit(main())