Preliminary work on resuming downloads whose connections have expired.

net/url_request/url_request.cc

Index: net/url_request/url_request.cc
diff --git a/net/url_request/url_request.cc b/net/url_request/url_request.cc
index b4f9d51f584fb7fb6a6040f06631702ec5b42ff8..10b1b12309112fbd39406bc17e3822ef061153ab 100644
--- a/net/url_request/url_request.cc
+++ b/net/url_request/url_request.cc
@@ -182,13 +182,14 @@ void URLRequest::SetExtraRequestHeaderByName(const string& name,
                                              const string& value,
                                              bool overwrite) {
   DCHECK(!is_pending_);
-  NOTREACHED() << "implement me!";
+  if (!overwrite || !extra_request_headers_.HasHeader(name))
+    extra_request_headers_.SetHeader(name, value);
 }
 
 void URLRequest::SetExtraRequestHeaders(
     const net::HttpRequestHeaders& headers) {
   DCHECK(!is_pending_);
-  extra_request_headers_ = headers;
+  extra_request_headers_.MergeFrom(headers);
 
   // NOTE: This method will likely become non-trivial once the other setters
   // for request headers are implemented.