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

net/spdy/platform/impl/spdy_string_impl.h

Index: net/spdy/platform/impl/spdy_string_impl.h
diff --git a/net/spdy/platform/impl/spdy_string_impl.h b/net/spdy/platform/impl/spdy_string_impl.h
new file mode 100644
index 0000000000000000000000000000000000000000..91a63367b14856d5d4dd23e7e86f6977a5dbbcab
--- /dev/null
+++ b/net/spdy/platform/impl/spdy_string_impl.h
@@ -0,0 +1,16 @@
+// Copyright (c) 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef NET_SPDY_PLATFORM_IMPL_SPDY_STRING_IMPL_H_
+#define NET_SPDY_PLATFORM_IMPL_SPDY_STRING_IMPL_H_
+
+#include <string>
+
+namespace net {
+
+using SpdyStringImpl = std::string;
+
+}  // namespace net
+
+#endif  // NET_SPDY_PLATFORM_IMPL_SPDY_STRING_IMPL_H_