servicediscovery: use NSNetService designated initializer in tests

Not doing this causes NSNetService to dealloc itself immediately,
which breaks the tests, on 10.15. Doing this is also objectively
correct. This fixes ServiceDiscoveryClientMacTest.* on 10.15.

Bug: 998755
Change-Id: I0f828073c74c3b9f5e611da01bc928c298efafa0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1827377
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700213}

chrome/browser/local_discovery/service_discovery_client_mac_unittest.mm

@@ -30,7 +30,7 @@
 @implementation TestNSNetService
 
 - (id)initWithData:(NSData*)data {
-  if ((self = [super init])) {
+  if ((self = [super initWithDomain:@"" type:@"_tcp." name:@"Test.123"])) {
     data_.reset([data retain]);
   }
   return self;