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

This regressed in r472101 wich made the network service pointer get initialized in non-test code earlier.

BUG=720374

Review-Url: https://codereview.chromium.org/2895353002
Cr-Commit-Position: refs/heads/master@{#473774}

content/public/common/network_service_test.mojom

@@ -13,5 +13,6 @@ struct Rule {
 interface NetworkServiceTest {
   // Adds the given host resolver rules in the process where the network
   // service is running.
-  AddRules(array<Rule> rules);
+  [Sync]
+  AddRules(array<Rule> rules) => ();
 };

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: