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

net/spdy/hpack/hpack_encoder.cc

Index: net/spdy/hpack/hpack_encoder.cc
diff --git a/net/spdy/hpack/hpack_encoder.cc b/net/spdy/hpack/hpack_encoder.cc
index a9d9df6181e6d985b279b45757e77563e0e14daa..48129d11819e9d5c031f67463c9afab9c176f7d1 100644
--- a/net/spdy/hpack/hpack_encoder.cc
+++ b/net/spdy/hpack/hpack_encoder.cc
@@ -17,8 +17,6 @@
 
 namespace net {
 
-using std::string;
-
 class HpackEncoder::RepresentationIterator {
  public:
   // |pseudo_headers| and |regular_headers| must outlive the iterator.
@@ -88,13 +86,13 @@ HpackEncoder::HpackEncoder(const HpackHuffmanTable& table)
 HpackEncoder::~HpackEncoder() {}
 
 void HpackEncoder::EncodeHeaderSet(const Representations& representations,
-                                   string* output) {
+                                   SpdyString* output) {
   RepresentationIterator iter(representations);
   EncodeRepresentations(&iter, output);
 }
 
 bool HpackEncoder::EncodeHeaderSet(const SpdyHeaderBlock& header_set,
-                                   string* output) {
+                                   SpdyString* output) {
   // Separate header set into pseudo-headers and regular headers.
   Representations pseudo_headers;
   Representations regular_headers;
@@ -139,7 +137,7 @@ size_t HpackEncoder::EstimateMemoryUsage() const {
 }
 
 void HpackEncoder::EncodeRepresentations(RepresentationIterator* iter,
-                                         string* output) {
+                                         SpdyString* output) {
   MaybeEmitTableSize();
   while (iter->HasNext()) {
     const auto header = iter->Next();
@@ -298,7 +296,7 @@ class HpackEncoder::Encoderator : public ProgressiveEncoder {
 
   // Encodes up to max_encoded_bytes of the current header block into the
   // given output string.
-  void Next(size_t max_encoded_bytes, string* output) override;
+  void Next(size_t max_encoded_bytes, SpdyString* output) override;
 
  private:
   HpackEncoder* encoder_;
@@ -335,7 +333,8 @@ HpackEncoder::Encoderator::Encoderator(const SpdyHeaderBlock& header_set,
   encoder_->MaybeEmitTableSize();
 }
 
-void HpackEncoder::Encoderator::Next(size_t max_encoded_bytes, string* output) {
+void HpackEncoder::Encoderator::Next(size_t max_encoded_bytes,
+                                     SpdyString* output) {
   SPDY_BUG_IF(!has_next_)
       << "Encoderator::Next called with nothing left to encode.";
   const bool use_compression = encoder_->enable_compression_;