Infra: Increase browser_tests sharding on mac-rel

Rolling back the bump to 25% and increasing sharding
Capacity looks good: http://shortn/_0ICs5GbtYR
However, build times have gotten much slower.

Bug: 1042757
Change-Id: Ifd058c01da1641aeea0adb04e3f5a6dff031aa81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596793Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Erik Staab <estaab@chromium.org>
Auto-Submit: Gary Tong <gatong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838235}

testing/buildbot/chromium.mac.json

@@ -3833,7 +3833,7 @@
         "test_id_prefix": "ninja://third_party/boringssl:boringssl_ssl_tests/"
       },
       {
-        "experiment_percentage": 25,
+        "experiment_percentage": 10,
         "isolate_profile_data": true,
         "merge": {
           "args": [],
@@ -3848,7 +3848,7 @@
             }
           ],
           "service_account": "chromium-tester@chops-service-accounts.iam.gserviceaccount.com",
-          "shards": 10
+          "shards": 20
         },
         "test": "browser_tests",
         "test_id_prefix": "ninja://chrome/test:browser_tests/"

testing/buildbot/test_suite_exceptions.pyl

@@ -520,7 +520,10 @@
         },
       },
       'Mac10.13 Tests':{
-        'experiment_percentage': 25 # https://crbug.com/1042757
+        'experiment_percentage': 10, # https://crbug.com/1042757
+        'swarming': {
+          'shards': 20,
+        },
       },
       'Mac10.13 Tests (dbg)': {
         'swarming': {