Do not override User-Agent header added by WebCore

net/http/http_request_headers.h

 // 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.
 //
 // HttpRequestHeaders manages the request headers.
 // It maintains these in a vector of header key/value pairs, thereby maintaining
 // the order of the headers.  This means that any lookups are linear time
 // operations.
 
 #ifndef NET_HTTP_HTTP_REQUEST_HEADERS_H_
@@ skipping 77 matching lines @@
   bool GetHeader(const base::StringPiece& key, std::string* out) const;
 
   // Clears all the headers.
   void Clear();
 
   // Sets the header value pair for |key| and |value|.  If |key| already exists,
   // then the header value is modified, but the key is untouched, and the order
   // in the vector remains the same.  When comparing |key|, case is ignored.
   void SetHeader(const base::StringPiece& key, const base::StringPiece& value);
 
+  // Sets the header value pair for |key| and |value|, if |key| does not exist.
+  // If |key| already exists, the call is a no-op.
+  // When comparing |key|, case is ignored.
+  void SetHeaderIfMissing(const base::StringPiece& key,
+                          const base::StringPiece& value);
+
   // Removes the first header that matches (case insensitive) |key|.
   void RemoveHeader(const base::StringPiece& key);
 
   // Parses the header from a string and calls SetHeader() with it.  This string
   // should not contain any CRLF.  As per RFC2616, the format is:
   //
   // message-header = field-name ":" [ field-value ]
   // field-name     = token
   // field-value    = *( field-content | LWS )
   // field-content  = <the OCTETs making up the field-value
@@ skipping 31 matching lines @@
   // Allow the copy construction and operator= to facilitate copying in
   // HttpRequestInfo.
   // TODO(willchan): Investigate to see if we can remove the need to copy
   // HttpRequestInfo.
   // DISALLOW_COPY_AND_ASSIGN(HttpRequestHeaders);
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_REQUEST_HEADERS_H_