Do not override User-Agent header added by WebCore

net/http/http_request_headers.cc

 // Copyright (c) 2010 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 "net/http/http_request_headers.h"
 
 #include "base/logging.h"
 #include "base/string_split.h"
 #include "base/string_util.h"
 #include "net/http/http_util.h"
@@ skipping 58 matching lines @@
   if (it == headers_.end())
     return false;
   out->assign(it->value);
   return true;
 }
 
 void HttpRequestHeaders::Clear() {
   headers_.clear();
 }
 
+void HttpRequestHeaders::SetHeaderIfMissing(const base::StringPiece& key,
+                                            const base::StringPiece& value) {
+  HeaderVector::iterator it = FindHeader(key);
+  if (it == headers_.end())
+    headers_.push_back(HeaderKeyValuePair(key.as_string(), value.as_string()));
+}
+
 void HttpRequestHeaders::SetHeader(const base::StringPiece& key,
                                    const base::StringPiece& value) {
   HeaderVector::iterator it = FindHeader(key);
   if (it != headers_.end())
     it->value = value.as_string();
   else
     headers_.push_back(HeaderKeyValuePair(key.as_string(), value.as_string()));
 }
 
 void HttpRequestHeaders::RemoveHeader(const base::StringPiece& key) {
@@ skipping 100 matching lines @@
        it != headers_.end(); ++it) {
     if (key.length() == it->key.length() &&
         !base::strncasecmp(key.data(), it->key.data(), key.length()))
       return it;
   }
 
   return headers_.end();
 }
 
 }  // namespace net