Remove window_manager_unittests from WebKit Linux Oilpan bot

testing/buildbot/chromium.webkit.json

 {
   "Linux ChromiumOS Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 1149 matching lines @@
       {
         "test": "views_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "webkit_unit_tests"
       },
       {
-        "test": "window_manager_unittests"
-      },
-      {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wm_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wtf_unittests"
@@ skipping 763 matching lines @@
         "name": "webkit_lint",
         "script": "webkit_lint.py"
       },
       {
         "name": "webkit_python_tests",
         "script": "webkit_python_tests.py"
       }
     ]
   }
 }