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

net/spdy/spdy_header_indexing.h

Index: net/spdy/spdy_header_indexing.h
diff --git a/net/spdy/spdy_header_indexing.h b/net/spdy/spdy_header_indexing.h
index a7d5c20fec5aec5c16e11935e3d637bbd0c83c9d..69cc08d23d2dbd6b183fba8a238b8529ab55df07 100644
--- a/net/spdy/spdy_header_indexing.h
+++ b/net/spdy/spdy_header_indexing.h
@@ -7,11 +7,11 @@
 
 #include <stdint.h>
 #include <memory>
-#include <string>
 #include <unordered_set>
 #include <utility>
 
 #include "net/base/net_export.h"
+#include "net/spdy/platform/api/spdy_string.h"
 #include "net/spdy/platform/api/spdy_string_piece.h"
 
 namespace net {
@@ -29,7 +29,7 @@ NET_EXPORT_PRIVATE extern int32_t FLAGS_gfe_spdy_tracking_set_bound;
 // UpdateSets to log the headers into both sets.
 class NET_EXPORT HeaderIndexing {
  public:
-  using HeaderSet = std::unordered_set<std::string>;
+  using HeaderSet = std::unordered_set<SpdyString>;
 
   HeaderIndexing();
   ~HeaderIndexing();
@@ -53,7 +53,7 @@ class NET_EXPORT HeaderIndexing {
 
  private:
   friend class test::HeaderIndexingPeer;
-  void TryInsertHeader(std::string&& header, HeaderSet* set, size_t bound);
+  void TryInsertHeader(SpdyString&& header, HeaderSet* set, size_t bound);
   // Headers to index.
   HeaderSet indexing_set_;
   // Headers seen so far.