Add OpenSSL BIO method that writes to a std::string.

crypto/openssl_bio_string.h

Index: crypto/openssl_bio_string.h
diff --git a/crypto/openssl_bio_string.h b/crypto/openssl_bio_string.h
new file mode 100644
index 0000000000000000000000000000000000000000..70d4331aa7687420db37aaf79cf16f80fb8082cb
--- /dev/null
+++ b/crypto/openssl_bio_string.h
@@ -0,0 +1,29 @@
+// Copyright 2014 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 CRYPTO_OPENSSL_BIO_STRING_H_
+#define CRYPTO_OPENSSL_BIO_STRING_H_
+
+#include <string>
+
+#include "crypto/crypto_export.h"
+
+// From <openssl/bio.h>
+typedef struct bio_st BIO;
+
+namespace crypto {
+
+// Creates a new BIO that can be used with OpenSSL's various output functions,
+// and which will write all output directly into |out|. This is primarily
+// intended as a utility to reduce the amount of copying and separate
+// allocations when performing extensive string modifications or streaming
+// within OpenSSL.
+//
+// Note: |out| must remain valid for the duration of the BIO.
+BIO* CRYPTO_EXPORT BIO_new_string(std::string* out);
+
+}  // namespace crypto
+
+#endif  // CRYPTO_OPENSSL_BIO_STRING_H_
+