Rework how isolates are generated in MB.

testing/buildbot/chromium.mac.json

Index: testing/buildbot/chromium.mac.json
diff --git a/testing/buildbot/chromium.mac.json b/testing/buildbot/chromium.mac.json
index 4a5ffab86db773387e4379407be5c6836d4aef47..ee1f847ddf4a17ced71a019145ca329d665b32ea 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -2,7 +2,6 @@
   "Mac GN": {
     "additional_compile_targets": [
       "accessibility_unittests",
-      "base_unittests",
       "cacheinvalidation_unittests",
       "cast_unittests",
       "cc_unittests",
@@ -25,12 +24,20 @@
       "skia_unittests",
       "sql_unittests",
       "sync_unit_tests",
-      "ui_base_unittests",
       "url_unittests"
     ],
     "gtest_tests": [
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "base_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "ui_base_unittests"
       }
     ]
   },
@@ -65,6 +72,9 @@
     ],
     "gtest_tests": [
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "base_unittests"
       }
     ]