Convert base_unittests to Swarming everywhere.

testing/buildbot/chromium.linux.json

 {
   "Android GN": {
     "additional_compile_targets": [
       "chrome_shell_apk",
       "mandoline:all"
     ],
     "gtest_tests": [
       {
         "test": "html_viewer_unittests"
       },
@@ skipping 67 matching lines @@
       },
       {
         "name": "telemetry_perf_unittests",
         "script": "telemetry_perf_unittests.py"
       }
     ]
   },
   "Cast Linux": {
     "gtest_tests": [
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "base_unittests"
       },
       {
         "test": "cacheinvalidation_unittests"
       },
       {
         "test": "cast_base_unittests"
       },
       {
         "test": "cast_media_unittests"
@@ skipping 1062 matching lines @@
       }
     ],
     "scripts": [
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   }
 }