Increasing sharding on browser_tests on mac to 40

Bug: 1042757
Change-Id: I720c371465c332d9128a61193d7b0af77461d94f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2630107
Auto-Submit: Gary Tong <gatong@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843795}

testing/buildbot/chromium.mac.json

@@ -3804,7 +3804,7 @@
             }
           ],
           "service_account": "chromium-tester@chops-service-accounts.iam.gserviceaccount.com",
-          "shards": 20
+          "shards": 40
         },
         "test": "browser_tests",
         "test_id_prefix": "ninja://chrome/test:browser_tests/"

testing/buildbot/test_suite_exceptions.pyl

@@ -517,7 +517,7 @@
       'Mac10.13 Tests':{
         'experiment_percentage': 10, # https://crbug.com/1042757
         'swarming': {
-          'shards': 20,
+          'shards': 40,
         },
       },
       'Mac10.13 Tests (dbg)': {