Converted accessibility_unittests to run on Swarming everywhere.

testing/buildbot/chromium.webkit.json

 {
   "Linux ChromiumOS Tests (1)": {
     "gtest_tests": [
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "accessibility_unittests"
       },
       {
         "test": "app_list_unittests"
       },
       {
         "test": "app_shell_unittests"
       },
       {
         "swarming": {
@@ skipping 153 matching lines @@
         "test": "wm_unittests"
       }
     ]
   },
   "Linux ChromiumOS Tests (2)": {
     "gtest_tests": []
   },
   "Linux ChromiumOS Tests (dbg)(1)": {
     "gtest_tests": [
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "accessibility_unittests"
       },
       {
         "test": "app_list_unittests"
       },
       {
         "test": "app_shell_unittests"
       },
       {
         "swarming": {
@@ skipping 696 matching lines @@
         "name": "webkit_lint",
         "script": "webkit_lint.py"
       },
       {
         "name": "webkit_python_tests",
         "script": "webkit_python_tests.py"
       }
     ]
   }
 }