Fix host resolver being initialized after network requests arrive to the network service.

content/shell/utility/shell_content_utility_client.cc

Index: content/shell/utility/shell_content_utility_client.cc
diff --git a/content/shell/utility/shell_content_utility_client.cc b/content/shell/utility/shell_content_utility_client.cc
index 69d156bba03461301c3357bdf5c7914563e53ecb..47a786080a5ccdd1994b9a3f871f0a9a9485b7c2 100644
--- a/content/shell/utility/shell_content_utility_client.cc
+++ b/content/shell/utility/shell_content_utility_client.cc
@@ -90,11 +90,13 @@ class NetworkServiceTestImpl : public mojom::NetworkServiceTest {
   ~NetworkServiceTestImpl() override = default;
 
   // mojom::NetworkServiceTest implementation.
-  void AddRules(std::vector<mojom::RulePtr> rules) override {
+  void AddRules(std::vector<mojom::RulePtr> rules,
+                AddRulesCallback callback) override {
     for (const auto& rule : rules) {
       test_host_resolver_.host_resolver()->AddRule(rule->host_pattern,
                                                    rule->replacement);
     }
+    std::move(callback).Run();
   }
 
  private: