Enable read-ahead on OpenSSL DTLS stream adapters.

Prevent multiple BIO reads when reading header and body but read from
internal OpenSSL buffer where possible.

BUG=chromium:447431
R=davidben@chromium.org, juberti@google.com

Review URL: https://webrtc-codereview.appspot.com/46319004

Cr-Commit-Position: refs/heads/master@{#9230}

webrtc/base/opensslstreamadapter.cc

@@ -739,6 +739,12 @@ int OpenSSLStreamAdapter::BeginSSL() {
   SSL_set_app_data(ssl_, this);
 
   SSL_set_bio(ssl_, bio, bio);  // the SSL object owns the bio now.
+  if (ssl_mode_ == SSL_MODE_DTLS) {
+    // Enable read-ahead for DTLS so whole packets are read from internal BIO
+    // before parsing.
+    // TODO(jbauch): Remove for BoringSSL when this is handled internally.
+    SSL_set_read_ahead(ssl_, 1);
+  }
 
   SSL_set_mode(ssl_, SSL_MODE_ENABLE_PARTIAL_WRITE |
                SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER);