We have ErrorReport::InitializeFromString, which just calls down to CertLoggerRequest::ParseFromString. There's a unittest covering this method, but the only invocation of this method appears to be CertificateReportingTest::MockReporter::SendExtendedReportingReport to extract the associated hostname.

The unit tests in error_report_unittest (with the exception of of ErrorReportTest:ParseSerializedReport) just seem to directly deserialize the CertLoggerRequest

One option is to rename ErrorReport::InitializeFromString to InitializeFromStringForTesting (e.g. enforcing it's test only), although alternatively, since the interface is such that it must maintain synchronicity with a remote server, having MockReporter::SendExtendedReportingReport just directly deserialize the CertLoggerRequest is another alternative.