discovery: use MOCK_TIME in unit tests

This knocks ~5% off the total runtime of unit_tests on my workstation, since
most tests in this suite otherwise take multiple seconds to run.

Bug: 1055824
Change-Id: Id911e97ce55efed11f6ee5fb34eaf5409426f19d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072660Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745047}

chrome/browser/local_discovery/service_discovery_client_unittest.cc

@@ -231,7 +231,8 @@ class ServiceDiscoveryTest : public ::testing::Test {
   net::MockMDnsSocketFactory socket_factory_;
   net::MDnsClientImpl mdns_client_;
   ServiceDiscoveryClientImpl service_discovery_client_;
-  base::test::SingleThreadTaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::TaskEnvironment::TimeSource::MOCK_TIME};
 };
 
 TEST_F(ServiceDiscoveryTest, AddRemoveService) {