Infra: Start re-enabling browser_tests on mac-rel

Starting at 10%.  Plan is to scale it up to 50% in the next couple days.

Bug: 1042757
Change-Id: I795f0a3aba74644e98818aaf44a5ebd6e22369dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591233
Auto-Submit: Gary Tong <gatong@chromium.org>
Commit-Queue: Gary Tong <gatong@chromium.org>
Commit-Queue: Erik Staab <estaab@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836871}

testing/buildbot/chromium.mac.json

@@ -3832,6 +3832,27 @@
         "test": "boringssl_ssl_tests",
         "test_id_prefix": "ninja://third_party/boringssl:boringssl_ssl_tests/"
       },
+      {
+        "experiment_percentage": 10,
+        "isolate_profile_data": true,
+        "merge": {
+          "args": [],
+          "script": "//testing/merge_scripts/standard_gtest_merge.py"
+        },
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "gpu": "none",
+              "os": "Mac-10.13.6"
+            }
+          ],
+          "service_account": "chromium-tester@chops-service-accounts.iam.gserviceaccount.com",
+          "shards": 10
+        },
+        "test": "browser_tests",
+        "test_id_prefix": "ninja://chrome/test:browser_tests/"
+      },
       {
         "args": [
           "--gtest_filter=-*UsingRealWebcam*"

testing/buildbot/test_suite_exceptions.pyl

@@ -462,7 +462,6 @@
       'CrWinAsan(dll)', # https://crbug.com/935598
       'linux-win_cross-rel',
       'ToTLinuxTSan',  # https://crbug.com/368525
-      'Mac10.13 Tests',  # https://crbug.com/1042757
       'Linux TSan Tests',  # https://crbug.com/368525
       'Win10 Tests x64 (dbg)',
     ],
@@ -532,6 +531,9 @@
           'shards': 30,
         },
       },
+      'Mac10.13 Tests':{
+        'experiment_percentage': 10 # https://crbug.com/1042757
+      },
       'Mac10.13 Tests (dbg)': {
         'swarming': {
           'shards': 20,