scheduler: Use a non-empty tracing category in test

BUG=706416
TBR=primiano@chromium.org

Review-Url: https://codereview.chromium.org/2786713002
Cr-Commit-Position: refs/heads/master@{#460484}

third_party/WebKit/Source/platform/scheduler/base/work_queue_unittest.cc

@@ -23,10 +23,10 @@ void NopTask() {}
 class WorkQueueTest : public testing::Test {
  public:
   void SetUp() override {
-    time_domain_.reset(new RealTimeDomain(""));
-    task_queue_ = make_scoped_refptr(
-        new TaskQueueImpl(nullptr, time_domain_.get(),
-                          TaskQueue::Spec(TaskQueue::QueueType::TEST), "", ""));
+    time_domain_.reset(new RealTimeDomain("test"));
+    task_queue_ = make_scoped_refptr(new TaskQueueImpl(
+        nullptr, time_domain_.get(),
+        TaskQueue::Spec(TaskQueue::QueueType::TEST), "test", "test"));
 
     work_queue_.reset(new WorkQueue(task_queue_.get(), "test",
                                     WorkQueue::QueueType::IMMEDIATE));