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

net/spdy/hpack/hpack_huffman_decoder.cc

Index: net/spdy/hpack/hpack_huffman_decoder.cc
diff --git a/net/spdy/hpack/hpack_huffman_decoder.cc b/net/spdy/hpack/hpack_huffman_decoder.cc
index d93306725a75c98d39e6d70aeb61ecd8a6b7dc98..49f9dae6ad27e62e888ee7895afdf345f5d19bc4 100644
--- a/net/spdy/hpack/hpack_huffman_decoder.cc
+++ b/net/spdy/hpack/hpack_huffman_decoder.cc
@@ -303,7 +303,7 @@ char HpackHuffmanDecoder::CanonicalToSource(HuffmanWord canonical) {
 // long strings, and a later portion dealing with the last few bytes of strings.
 // TODO(jamessynge): Determine if that is worth it by adding some counters to
 // measure the distribution of string sizes seen in practice.
-bool HpackHuffmanDecoder::DecodeString(HpackInputStream* in, std::string* out) {
+bool HpackHuffmanDecoder::DecodeString(HpackInputStream* in, SpdyString* out) {
   out->clear();
 
   // Load |bits| with the leading bits of the input stream, left justified