Disable synchronous XMLHttpRequests in platform app documents.

chrome/browser/extensions/extension_webkit_preferences.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/extensions/extension_webkit_preferences.h"
 
 #include "base/command_line.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/extensions/extension.h"
 #include "webkit/glue/webpreferences.h"
@@ skipping 20 matching lines @@
     // memory impact of extensions. See crbug.com/123935. This also "works
     // around" crbug.com/123935
     if (render_view_type == chrome::VIEW_TYPE_EXTENSION_BACKGROUND_PAGE) {
       webkit_prefs->force_compositing_mode = false;
     }
   }
 
   if (extension->is_platform_app()) {
     webkit_prefs->databases_enabled = false;
     webkit_prefs->local_storage_enabled = false;
+    webkit_prefs->sync_xhr_in_documents_enabled = false;
   }
 
   // Enable WebGL features that regular pages can't access, since they add
   // more risk of fingerprinting.
   webkit_prefs->privileged_webgl_extensions_enabled = true;
 
   // If this is a component extension, then apply the same poliy for
   // accelerated compositing as for chrome: URLs (from
   // WebContents::GetWebkitPrefs).  This is important for component extensions
   // like the file manager which are sometimes loaded using chrome: URLs and
   // sometimes loaded with chrome-extension: URLs - we should expect the
   // performance characteristics to be similar in both cases.
   const CommandLine& command_line = *CommandLine::ForCurrentProcess();
   if (extension->location() == extensions::Extension::COMPONENT &&
       !command_line.HasSwitch(switches::kAllowWebUICompositing)) {
     webkit_prefs->accelerated_compositing_enabled = false;
     webkit_prefs->accelerated_2d_canvas_enabled = false;
   }
 }
 
 }  // extension_webkit_preferences