Create stubs for system certificate validation.

net/base/single_request_cert_verifier.cc

Index: net/base/single_request_cert_verifier.cc
diff --git a/net/base/single_request_cert_verifier.cc b/net/base/single_request_cert_verifier.cc
index 60f9f1c8be0a87131e849cf58a4960c22d28e343..58017b492e519e470207402d7e3d358feffbcaab 100644
--- a/net/base/single_request_cert_verifier.cc
+++ b/net/base/single_request_cert_verifier.cc
@@ -35,10 +35,6 @@ int SingleRequestCertVerifier::Verify(X509Certificate* cert,
   // Should not be already in use.
   DCHECK(!cur_request_ && cur_request_callback_.is_null());
 
-  // Do a synchronous verification.

[Ryan Sleevi, 2012/03/03 05:01:50]

I checked every user of SingleRequestCertVerifier, and they all provide
callbacks. Plus, I believe SingleRequestCertVerifier should be ignorant about
sync vs async here.

[wtc, 2012/03/06 23:10:14]

I guess we can replace this with a DCHECK?  The code below
won't work if cert_verifier_->Verify() returns ERR_IO_PENDING
but callback is null.

-  if (callback.is_null())
-    return cert->Verify(hostname, flags, crl_set, verify_result);
-
   CertVerifier::RequestHandle request = NULL;
 
   // We need to be notified of completion before |callback| is called, so that