Revert of Remove remnants of DHE support.

net/socket/ssl_client_socket_impl.cc

Index: net/socket/ssl_client_socket_impl.cc
diff --git a/net/socket/ssl_client_socket_impl.cc b/net/socket/ssl_client_socket_impl.cc
index c993fa8fb23da2d1d58c35731b3801a19283ff11..4be80d76f56120d554204cdb41592a77e91709a8 100644
--- a/net/socket/ssl_client_socket_impl.cc
+++ b/net/socket/ssl_client_socket_impl.cc
@@ -993,11 +993,23 @@
   SSL_clear_mode(ssl_.get(), mode.clear_mask);
 
   // Use BoringSSL defaults, but disable HMAC-SHA256 and HMAC-SHA384 ciphers
-  // (note that SHA256 and SHA384 only select legacy CBC ciphers).
-  std::string command("ALL:!SHA256:!SHA384:!kDHE:!aPSK:!RC4");
+  // (note that SHA256 and SHA384 only select legacy CBC ciphers). Also disable
+  // DHE_RSA_WITH_AES_256_GCM_SHA384. Historically, AES_256_GCM was not
+  // supported. As DHE is being deprecated, don't add a cipher only to remove
+  // it immediately.
+  //
+  // TODO(davidben): Remove the DHE_RSA_WITH_AES_256_GCM_SHA384 exclusion when
+  // the DHEEnabled administrative policy expires.
+  std::string command(
+      "ALL:!SHA256:!SHA384:!DHE-RSA-AES256-GCM-SHA384:!aPSK:!RC4");
 
   if (ssl_config_.require_ecdhe)
     command.append(":!kRSA:!kDHE");
+
+  if (!ssl_config_.deprecated_cipher_suites_enabled) {
+    // Only offer DHE on the second handshake. https://crbug.com/538690
+    command.append(":!kDHE");
+  }
 
   // Additionally disable HMAC-SHA1 ciphers in ECDSA. These are the remaining
   // CBC-mode ECDSA ciphers.
@@ -1147,6 +1159,16 @@
       SSL_session_reused(ssl_.get())) {
     UMA_HISTOGRAM_EXACT_LINEAR("Net.SSLSessionConcurrentLookupCount",
                                ssl_session_cache_lookup_count_, 20);
+  }
+
+  // DHE is offered on the deprecated cipher fallback and then rejected
+  // afterwards. This is to aid in diagnosing connection failures because a
+  // server requires DHE ciphers.
+  //
+  // TODO(davidben): A few releases after DHE's removal, remove this logic.
+  if (!ssl_config_.dhe_enabled &&
+      SSL_CIPHER_is_DHE(SSL_get_current_cipher(ssl_.get()))) {
+    return ERR_SSL_OBSOLETE_CIPHER;
   }
 
   // Check that if token binding was negotiated, then extended master secret