Fix DCHECK in debug build when navigating to an unknown chrome://theme/ URL.

content/browser/webui/url_data_manager_backend.h

 // 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.
 
 #ifndef CONTENT_BROWSER_WEBUI_URL_DATA_MANAGER_BACKEND_H_
 #define CONTENT_BROWSER_WEBUI_URL_DATA_MANAGER_BACKEND_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 34 matching lines @@
   // be set for incognito profiles. Called on the UI thread.
   CONTENT_EXPORT static std::unique_ptr<
       net::URLRequestJobFactory::ProtocolHandler>
   CreateProtocolHandler(content::ResourceContext* resource_context,
                         bool is_incognito,
                         ChromeBlobStorageContext* blob_storage_context);
 
   // Adds a DataSource to the collection of data sources.
   void AddDataSource(URLDataSourceImpl* source);
 
-  // DataSource invokes this. Sends the data to the URLRequest.
+  // DataSource invokes this. Sends the data to the URLRequest. |bytes| may be
+  // null, which signals an error handling the request.
   void DataAvailable(RequestID request_id, base::RefCountedMemory* bytes);
 
   static net::URLRequestJob* Factory(net::URLRequest* request,
                                      const std::string& scheme);
 
  private:
   friend class URLRequestChromeJob;
 
   typedef std::map<std::string,
       scoped_refptr<URLDataSourceImpl> > DataSourceMap;
@@ skipping 40 matching lines @@
 
 // Creates protocol handler for chrome-devtools://. |is_incognito| should be
 // set for incognito profiles.
 net::URLRequestJobFactory::ProtocolHandler*
 CreateDevToolsProtocolHandler(content::ResourceContext* resource_context,
                               bool is_incognito);
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_WEBUI_URL_DATA_MANAGER_BACKEND_H_