Add logic to disable default gpu dimension.

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 427 matching lines @@
 
 
 def get_swarming_dimension(dimension, device_affinity):
   complete_dimension = {
     'id': dimension['device_ids'][device_affinity],
     'os': dimension['os'],
     'pool': 'Chrome-perf',
   }
   if 'gpu' in dimension:
     complete_dimension['gpu'] = dimension['gpu']
+  else:
+    complete_dimension['gpu'] = None
   return complete_dimension
 
 
 def generate_cplusplus_isolate_script_test(dimension):
   return [
     generate_isolate_script_entry(
         [get_swarming_dimension(dimension, shard)], [], name, name)
     for name, shard in dimension['perf_tests']
   ]
 
@@ skipping 204 matching lines @@
   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())