Update generate_perf_json.py with Mac Mini 8GB builder.

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 394 matching lines @@
     swarming=[
       {
        'gpu': '8086:1626',
        'os': 'Mac-10.11',
        'device_ids': [
            'build123-b1', 'build124-b1',
            'build125-b1', 'build126-b1', 'build127-b1'
           ]
       }
     ])
+  waterfall = add_tester(
+    waterfall, 'Mac Mini 8GB 10.12 Perf',
+    'chromium-rel-mac12-mini-8gb', 'mac',
+    swarming=[
+      {
+       'gpu': '8086:0a26',
+       'os': 'Mac-10.12',
+       'device_ids': [
+           'build24-b1', 'build25-b1',
+           'build26-b1', 'build27-b1', 'build28-b1'
+          ]
+      }
+    ])
 
   waterfall = add_tester(
     waterfall, 'Linux Perf', 'linux-release', 'linux',
     swarming=[
       {
        'gpu': '102b:0534',
        'os': 'Ubuntu-14.04',
        'device_ids': [
            'build148-m1', 'build149-m1',
            'build150-m1', 'build151-m1', 'build152-m1'
@@ skipping 329 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())