Mock implementation of overridePreference.

chrome/test/data/layout_tests/layout_test_controller.html

 <script>
 function LayoutTestController() {
   this.dump_as_text_ = false;
   this.wait_until_done_ = false;
 
   this.dumpAsText = function () {
     this.dump_as_text_ = true;
   };
 
   this.waitUntilDone = function () {
     this.wait_until_done_ = true;
   };
 
   this.notifyDone = function () {
     // For storage events, it actually takes 2 rounds of yielding for everything
     // to complete.
     setTimeout(layoutTestController.DelayedNotifyDone1, 0);
   };
 
+  this.overridePreference = function () {
+    // WebSocket tests call this function by preference name
+    // 'WebKitHixie76WebSocketProtocolEnabled' to switch its protocol version.
+    // In Chromium, the value must be 0 because it doesn't support old one.
+  };
+
   this.DelayedNotifyDone1 = function () {
     setTimeout(layoutTestController.DelayedNotifyDone2, 0);
   };
 
   this.DelayedNotifyDone2 = function () {
     var token = encodeURIComponent(document.body.innerText);
     document.cookie = "%COOKIE%=" + token;
   };
 
   this.OnEvent = function () {
     // HACK: dumpAsText() is always called after waitUntilDone() and all tests
     //       that are run within the UI test framework either call
     //       waitUntilDone() or dumpAsText().  So we'll simply keep polling
     //       until we see one or the other.
     if (layoutTestController.dump_as_text_) {
       if (!layoutTestController.wait_until_done_)
         layoutTestController.notifyDone();
     } else {
       setTimeout(layoutTestController.OnEvent, 200);
     }
   };
 }
 
 window.layoutTestController = new LayoutTestController();
 window.addEventListener('load', layoutTestController.OnEvent, false);
 </script>