Add SpdyString alias for std::string in net/spdy.

net/spdy/spdy_proxy_client_socket.cc

Index: net/spdy/spdy_proxy_client_socket.cc
diff --git a/net/spdy/spdy_proxy_client_socket.cc b/net/spdy/spdy_proxy_client_socket.cc
index 9b9ce555640d1910b95b709a58449d3d4d28328b..24805e1a960ebd928b0ecdfec552b2ca9660ccce 100644
--- a/net/spdy/spdy_proxy_client_socket.cc
+++ b/net/spdy/spdy_proxy_client_socket.cc
@@ -33,7 +33,7 @@ namespace net {
 
 SpdyProxyClientSocket::SpdyProxyClientSocket(
     const base::WeakPtr<SpdyStream>& spdy_stream,
-    const std::string& user_agent,
+    const SpdyString& user_agent,
     const HostPortPair& endpoint,
     const HostPortPair& proxy_server,
     const NetLogWithSource& source_net_log,
@@ -351,7 +351,7 @@ int SpdyProxyClientSocket::DoSendRequest() {
     auth_->AddAuthorizationHeader(&authorization_headers);
   }
 
-  std::string request_line;
+  SpdyString request_line;
   BuildTunnelRequest(endpoint_, authorization_headers, user_agent_,
                      &request_line, &request_.extra_headers);