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

net/spdy/hpack/hpack_input_stream.cc

Index: net/spdy/hpack/hpack_input_stream.cc
diff --git a/net/spdy/hpack/hpack_input_stream.cc b/net/spdy/hpack/hpack_input_stream.cc
index 0d0dfc2fcb602cbc45b948d54cbe7f8211a5d6c6..9cc27f5ae7e87a6d21ecef574bf02b4f5604bdc7 100644
--- a/net/spdy/hpack/hpack_input_stream.cc
+++ b/net/spdy/hpack/hpack_input_stream.cc
@@ -12,8 +12,6 @@
 
 namespace net {
 
-using std::string;
-
 HpackInputStream::HpackInputStream(SpdyStringPiece buffer)
     : buffer_(buffer),
       bit_offset_(0),
@@ -135,7 +133,7 @@ bool HpackInputStream::DecodeNextIdentityString(SpdyStringPiece* str) {
   return true;
 }
 
-bool HpackInputStream::DecodeNextHuffmanString(string* str) {
+bool HpackInputStream::DecodeNextHuffmanString(SpdyString* str) {
   uint32_t encoded_size = 0;
   if (!DecodeNextUint32(&encoded_size)) {
     if (!need_more_data_) {