Depending on the test configuration, some browser tests may be communicating with the production autofill server.

Many browser tests set up (Test|Fake|Mock)UrlFetcherFactory instances for the duration of the test, these tests may be generating (Test|Fake|Mock)UrlFetcher request, depending on exactly what's involved in the test. This is most likely benign.

Other tests use the embedded test server to server pages. These tests may (depending on the specific test) generate real URLFetcher requests to the production autofill servers. It's not immediately clear whether the I/O thread services these requests, but likely it does.

Modify the browser test base classes to disable the AutofillDownloadManager by default or to route it's request to a fake endpoint.