Add Content-Type header to net::ReportSender reports

chrome/browser/ssl/chrome_expect_ct_reporter.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 "chrome/browser/ssl/chrome_expect_ct_reporter.h"
 
 #include <string>
 
 #include "base/base64.h"
 #include "base/command_line.h"
@@ skipping 158 matching lines @@
   report.Set("valid-scts", std::move(valid_scts));
 
   std::string serialized_report;
   if (!base::JSONWriter::Write(report, &serialized_report)) {
     LOG(ERROR) << "Failed to serialize Expect CT report";
     return;
   }
 
   UMA_HISTOGRAM_BOOLEAN("SSL.ExpectCTReportSendingAttempt", true);
 
-  report_sender_->Send(report_uri, serialized_report);
+  report_sender_->Send(report_uri, "application/json; charset=utf-8",
+                       serialized_report);
 }