Convert content_unittests to run exclusively on Swarming

testing/buildbot/chromium.webkit.json

 {
   "Linux ChromiumOS Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 51 matching lines @@
         },
         "test": "content_browsertests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "content_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "content_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "crypto_unittests"
       },
       {
         "test": "dbus_unittests"
@@ skipping 979 matching lines @@
         "name": "webkit_lint",
         "script": "webkit_lint.py"
       },
       {
         "name": "webkit_python_tests",
         "script": "webkit_python_tests.py"
       }
     ]
   }
 }