Add browser tags to all of our tests.

test/runner/browser/expanded_reporter_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 @TestOn("vm")
 
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
@@ skipping 17 matching lines @@
       "-r", "expanded",
       "-p", "content-shell",
       "-p", "vm",
       "-j", "1",
       "test.dart"
     ]);
 
     test.stdout.fork().expect(consumeThrough(contains("[VM]")));
     test.stdout.expect(consumeThrough(contains("[Dartium Content Shell]")));
     test.shouldExit(0);
-  });
+  }, tags: ['content-shell']);
 }