Rework how isolates are generated in MB.

testing/buildbot/chromium.linux.json

Index: testing/buildbot/chromium.linux.json
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index e2edde69bc12786d3384238929a14b7f9f3e4c03..436e94e9c0813a65a1a85b27fc34a75a01e5adf9 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -333,6 +333,9 @@
         "test": "sync_unit_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "ui_base_unittests"
       },
       {
@@ -416,6 +419,20 @@
       "url_unittests",
       "views_unittests",
       "wm_unittests"
+    ],
+    "gtest_tests": [
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "base_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "ui_base_unittests"
+      }
     ]
   },
   "Linux GN Clobber": {