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

net/spdy/http2_frame_decoder_adapter.cc

Index: net/spdy/http2_frame_decoder_adapter.cc
diff --git a/net/spdy/http2_frame_decoder_adapter.cc b/net/spdy/http2_frame_decoder_adapter.cc
index 5e21f7840f77c0ec59d5205efce969f96ded964f..8b74e6ec38532f3419f3f019823e479502201b42 100644
--- a/net/spdy/http2_frame_decoder_adapter.cc
+++ b/net/spdy/http2_frame_decoder_adapter.cc
@@ -13,7 +13,6 @@
 
 #include <cstdint>
 #include <cstring>
-#include <string>
 #include <utility>
 
 #include "base/logging.h"
@@ -28,6 +27,7 @@
 #include "net/spdy/hpack/hpack_decoder_interface.h"
 #include "net/spdy/hpack/hpack_header_table.h"
 #include "net/spdy/platform/api/spdy_estimate_memory_usage.h"
+#include "net/spdy/platform/api/spdy_string.h"
 #include "net/spdy/spdy_alt_svc_wire_format.h"
 #include "net/spdy/spdy_bug_tracker.h"
 #include "net/spdy/spdy_frame_builder.h"
@@ -35,8 +35,6 @@
 #include "net/spdy/spdy_headers_handler_interface.h"
 #include "net/spdy/spdy_protocol.h"
 
-using std::string;
-
 namespace net {
 
 namespace {
@@ -940,8 +938,8 @@ class Http2DecoderAdapter : public SpdyFramerDecoderAdapter,
   base::Optional<size_t> opt_pad_length_;
 
   // Temporary buffers for the AltSvc fields.
-  string alt_svc_origin_;
-  string alt_svc_value_;
+  SpdyString alt_svc_origin_;
+  SpdyString alt_svc_value_;
 
   // Listener used if we transition to an error state; the listener ignores all
   // the callbacks.