Have MimeSniffingResourceHandler call WillStartRequest on new Handlers.

content/browser/loader/intercepting_resource_handler.cc

 // Copyright 2016 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 "content/browser/loader/intercepting_resource_handler.h"
 
 #include "base/logging.h"
 #include "base/strings/string_util.h"
 #include "content/public/common/resource_response.h"
 #include "net/base/io_buffer.h"
+#include "net/url_request/url_request.h"
 
 namespace content {
 
 InterceptingResourceHandler::InterceptingResourceHandler(
     std::unique_ptr<ResourceHandler> next_handler,
     net::URLRequest* request)
     : LayeredResourceHandler(request, std::move(next_handler)),
       state_(State::STARTING),
       first_read_buffer_size_(0) {}
 
@@ skipping 133 matching lines @@
   payload_for_old_handler_ = std::string();
   DCHECK(!defer_ignored);
 
   // Finalize the request.
   net::URLRequestStatus status(net::URLRequestStatus::SUCCESS, 0);
   next_handler_->OnResponseCompleted(status, &defer_ignored);
   DCHECK(!defer_ignored);
 }
 
 }  // namespace content