Convert content_unittests to run exclusively on Swarming

testing/buildbot/chromium.chromiumos.json

 {
   "Linux ChromiumOS GN": {
     "additional_compile_targets": [
       "accessibility_unittests",
       "app_list_unittests",
       "aura_unittests",
       "browser_tests",
       "cacheinvalidation_unittests",
       "chromedriver_unittests",
       "chromeos_unittests",
@@ skipping 144 matching lines @@
         "test": "components_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true,
           "shards": 2
         },
         "test": "content_browsertests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "content_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "crypto_unittests"
       },
       {
         "test": "dbus_unittests"
@@ skipping 490 matching lines @@
       },
       {
         "test": "views_unittests"
       },
       {
         "test": "wm_unittests"
       }
     ]
   }
 }