Add field trial config for DnsOverHttps experiment.

Bug: 985589
Change-Id: I0431adf4c4d26ab94dafd34f2a6bc6e372d8d2d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890817Reviewed-by: Eric Orth <ericorth@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Katharine Daly <dalyk@google.com>
Cr-Commit-Position: refs/heads/master@{#711401}

chrome/browser/net/system_network_context_manager_browsertest.cc

@@ -69,7 +69,11 @@ void RunStubResolverConfigTests(bool async_dns_feature_enabled) {
   GetStubResolverConfig(&insecure_stub_resolver_enabled, &secure_dns_mode,
                         &dns_over_https_servers);
   EXPECT_EQ(async_dns_feature_enabled, insecure_stub_resolver_enabled);
-  EXPECT_EQ(net::DnsConfig::SecureDnsMode::OFF, secure_dns_mode);
+  if (base::FeatureList::IsEnabled(features::kDnsOverHttps)) {
+    EXPECT_EQ(net::DnsConfig::SecureDnsMode::AUTOMATIC, secure_dns_mode);
+  } else {
+    EXPECT_EQ(net::DnsConfig::SecureDnsMode::OFF, secure_dns_mode);
+  }
   EXPECT_FALSE(dns_over_https_servers.has_value());
 
   std::string good_post_template = "https://foo.test/";

testing/variations/fieldtrial_testing_config.json

@@ -2011,6 +2011,24 @@
             ]
         }
     ],
+    "DnsOverHttps": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_crbug1006105",
+                    "enable_features": [
+                        "DnsOverHttps"
+                    ]
+                }
+            ]
+        }
+    ],
     "DownloadHomeMoreButton": [
         {
             "platforms": [