Backport some tests in /chrome/browser/local_discovery to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/chrome/browser/local_discovery.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=weili@chromium.org

Bug: 891670
Change-Id: I2d20ba890d0416f9812b85efd87608ab7cb0ec26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786739
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695125}

chrome/browser/local_discovery/local_domain_resolver_unittest.cc

@@ -103,7 +103,7 @@ class LocalDomainResolverTest : public testing::Test {
 
   net::MockMDnsSocketFactory socket_factory_;
   net::MDnsClientImpl mdns_client_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 TEST_F(LocalDomainResolverTest, ResolveDomainA) {

chrome/browser/local_discovery/service_discovery_client_unittest.cc

@@ -231,7 +231,7 @@ class ServiceDiscoveryTest : public ::testing::Test {
   net::MockMDnsSocketFactory socket_factory_;
   net::MDnsClientImpl mdns_client_;
   ServiceDiscoveryClientImpl service_discovery_client_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 TEST_F(ServiceDiscoveryTest, AddRemoveService) {