Add test for posting a single task to the worker pool

src/libplatform/task-queue.h

Index: src/libplatform/task-queue.h
diff --git a/src/libplatform/task-queue.h b/src/libplatform/task-queue.h
index 24b68da04b661eb9cf5be3ba9a18fb1788431f3a..5239cdac401d525773efd5c36380409776d07825 100644
--- a/src/libplatform/task-queue.h
+++ b/src/libplatform/task-queue.h
@@ -10,6 +10,7 @@
 #include "src/base/macros.h"
 #include "src/base/platform/mutex.h"
 #include "src/base/platform/semaphore.h"
+#include "testing/gtest/include/gtest/gtest_prod.h"
 
 namespace v8 {
 
@@ -33,6 +34,10 @@ class TaskQueue {
   void Terminate();
 
  private:
+  FRIEND_TEST(WorkerThreadTest, PostSingleTask);
+
+  void BlockUntilQueueEmptyForTesting();
+
   base::Semaphore process_queue_semaphore_;
   base::Mutex lock_;
   std::queue<Task*> task_queue_;