HostResolver: don't interpret NULL callback argument as a request to do synchronous resolution.

net/base/mock_host_resolver.cc

Index: net/base/mock_host_resolver.cc
diff --git a/net/base/mock_host_resolver.cc b/net/base/mock_host_resolver.cc
index f822bdea0b76b1e9b49aa505364764a5668fc29b..6b2aa4c5c4516bb315d14351ee9638a342950246 100644
--- a/net/base/mock_host_resolver.cc
+++ b/net/base/mock_host_resolver.cc
@@ -112,6 +112,12 @@ int MockHostResolverBase::Resolve(const RequestInfo& info,
   return impl_->Resolve(info, addresses, callback, out_req, net_log);
 }
 
+int MockHostResolverBase::ResolveFromCache(const RequestInfo& info,
+                                           AddressList* addresses,
+                                           const BoundNetLog& net_log) {
+  return impl_->ResolveFromCache(info, addresses, net_log);
+}
+
 void MockHostResolverBase::CancelRequest(RequestHandle req) {
   impl_->CancelRequest(req);
 }