scheduler: Start running WebThread unit tests again

Looks like these were accidentally removed when the scheduler was turned into
a component.

BUG=477690

Review URL: https://codereview.chromium.org/1112523003

Cr-Commit-Position: refs/heads/master@{#327271}
diff --git a/components/scheduler/BUILD.gn b/components/scheduler/BUILD.gn
index 2d00aef..969af30 100644
--- a/components/scheduler/BUILD.gn
+++ b/components/scheduler/BUILD.gn
@@ -42,9 +42,11 @@
     "child/task_queue_manager_unittest.cc",
     "child/test_time_source.cc",
     "child/test_time_source.h",
+    "child/webthread_impl_for_worker_scheduler_unittest.cc",
     "child/worker_scheduler_impl_unittest.cc",
     "renderer/deadline_task_runner_unittest.cc",
     "renderer/renderer_scheduler_impl_unittest.cc",
+    "renderer/webthread_impl_for_renderer_scheduler_unittest.cc",
   ]
 
   deps = [