Infra: Re-enable browser_tests on mac-rel

Bumping up to 25%  Capacity looks good so far: http://shortn/_0ICs5GbtYR

Bug: 1042757
Change-Id: I9ce89c7f8f1c0034660ab64dd58299ffe0d3e80a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593596
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>
Cr-Commit-Position: refs/heads/master@{#837427}

testing/buildbot/chromium.mac.json

@@ -3833,7 +3833,7 @@
         "test_id_prefix": "ninja://third_party/boringssl:boringssl_ssl_tests/"
       },
       {
-        "experiment_percentage": 10,
+        "experiment_percentage": 25,
         "isolate_profile_data": true,
         "merge": {
           "args": [],

testing/buildbot/test_suite_exceptions.pyl

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