Rework how isolates are generated in MB.

testing/buildbot/chromium.win.json

Index: testing/buildbot/chromium.win.json
diff --git a/testing/buildbot/chromium.win.json b/testing/buildbot/chromium.win.json
index 038dbc4af0c4b4b669cbd0546bb3fef72bf44143..3eec2d08af56e0922bc284ee2758e562f00b1d86 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -1129,19 +1129,30 @@
       "sql_unittests",
       "sync_integration_tests",
       "sync_unit_tests",
-      "ui_base_unittests",
       "url_unittests",
       "views_unittests",
       "wm_unittests"
     ],
     "gtest_tests": [
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "base_unittests"
+      },
+      {
         "test": "content_browsertests"
       },
       {
         "test": "events_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "ui_base_unittests"
+      },
+      {
         "test": "ui_touch_selection_unittests"
       }
     ]
@@ -1215,12 +1226,10 @@
       "components_browsertests",
       "components_unittests",
       "compositor_unittests",
-      "content_browsertests",
       "content_unittests",
       "courgette_unittests",
       "crypto_unittests",
       "device_unittests",
-      "events_unittests",
       "extensions_browsertests",
       "extensions_unittests",
       "gcm_unit_tests",
@@ -1245,11 +1254,32 @@
       "sql_unittests",
       "sync_integration_tests",
       "sync_unit_tests",
-      "ui_base_unittests",
-      "ui_touch_selection_unittests",
       "url_unittests",
       "views_unittests",
       "wm_unittests"
+    ],
+    "gtest_tests": [
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "base_unittests"
+      },
+      {
+        "test": "content_browsertests"
+      },
+      {
+        "test": "events_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "ui_base_unittests"
+      },
+      {
+        "test": "ui_touch_selection_unittests"
+      }
     ]
   },
   "XP Tests (1)": {