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

crypto/crypto.gyp

Index: crypto/crypto.gyp
diff --git a/crypto/crypto.gyp b/crypto/crypto.gyp
index ec1e2782a1a3582bee006af43da7c1876e4f0556..a14b30c9a1405adbb4d14a08dd088e3545060d5d 100644
--- a/crypto/crypto.gyp
+++ b/crypto/crypto.gyp
@@ -132,6 +132,8 @@
               'ec_signature_creator_openssl.cc',
               'encryptor_openssl.cc',
               'hmac_openssl.cc',
+              'openssl_bio_string.cc',
+              'openssl_bio_string.h',
               'openssl_util.cc',
               'openssl_util.h',
               'rsa_private_key_openssl.cc',
@@ -158,6 +160,7 @@
         'hkdf_unittest.cc',
         'hmac_unittest.cc',
         'nss_util_unittest.cc',
+        'openssl_bio_string_unittest.cc',
         'p224_unittest.cc',
         'p224_spake_unittest.cc',
         'random_unittest.cc',
@@ -209,6 +212,10 @@
             'nss_util_unittest.cc',
             'rsa_private_key_nss_unittest.cc',
           ],
+        }, {
+          'sources!': [
+            'openssl_bio_string_unittest.cc',
+          ],
         }],
       ],
     },