HttpStreamPool: Add debug instrumentation

There seems a case where we don't set `most_recent_tcp_error_` after
all TCP attempts are failed. Reported minidumps don't contain enough
information to identify the cause. This CL adds some
base::debug::Alias to identify the cause.

Bug: 403373872
Change-Id: I7161d2e8703c58d6640b67cf638157d4481447e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6395996
Reviewed-by: Nidhi Jaju <nidhijaju@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437943}

net/http/http_stream_pool_attempt_manager.cc

@@ -1322,7 +1322,19 @@ void HttpStreamPool::AttemptManager::MaybeAttemptConnection(
               TcpBasedAttemptState::kAllEndpointsFailed &&
           !quic_task_) {
         // Tried all endpoints.
-        DCHECK(most_recent_tcp_error_.has_value());
+        // TODO(crbug.com/403373872): Replace the following `if` with CHECK()
+        // once we identify the root cause.
+        if (!most_recent_tcp_error_.has_value()) {
+          const bool is_svcb_optional = IsSvcbOptional();
+          ConnectionAttempts connection_attempts = connection_attempts_;
+          std::vector<ServiceEndpoint> endpoints =
+              service_endpoint_request_->GetEndpointResults();
+          base::debug::Alias(&is_svcb_optional);
+          base::debug::Alias(&connection_attempts_);
+          base::debug::Alias(&endpoints);
+          DEBUG_ALIAS_FOR_GURL(url_buf, stream_key().destination().GetURL());
+          NOTREACHED();
+        }
         HandleFinalError(*most_recent_tcp_error_);
       }
       return;