Remove the concept of user stylesheets.

Source/testing/runner/WebPreferences.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 37 matching lines @@
     experimentalCSSRegionsEnabled = true;
     experimentalCSSGridLayoutEnabled = true;
     javaEnabled = false;
     javaScriptCanAccessClipboard = true;
     javaScriptCanOpenWindowsAutomatically = true;
     supportsMultipleWindows = true;
     javaScriptEnabled = true;
     loadsImagesAutomatically = true;
     offlineWebApplicationCacheEnabled = true;
     pluginsEnabled = true;
-    userStyleSheetLocation = WebURL();
     caretBrowsingEnabled = false;
 
     // Allow those layout tests running as local files, i.e. under
     // LayoutTests/http/tests/local, to access http server.
     allowUniversalAccessFromFileURLs = true;
 
 #ifdef __APPLE__
     editingBehavior = WebSettings::EditingBehaviorMac;
 #else
     editingBehavior = WebSettings::EditingBehaviorWin;
 #endif
 
     tabsToLinks = false;
     hyperlinkAuditingEnabled = false;
     cssCustomFilterEnabled = false;
     shouldRespectImageOrientation = false;
     asynchronousSpellCheckingEnabled = false;
 }
 
 }