Add test for posting a single task to the worker pool

src/libplatform/task-queue.cc

Index: src/libplatform/task-queue.cc
diff --git a/src/libplatform/task-queue.cc b/src/libplatform/task-queue.cc
index 0a630ed3c394aff140f75e8ec10b660b6538ccd1..ada13d9fe947301a2bff2d202916eb2aee1efb10 100644
--- a/src/libplatform/task-queue.cc
+++ b/src/libplatform/task-queue.cc
@@ -5,6 +5,8 @@
 #include "src/libplatform/task-queue.h"
 
 #include "src/base/logging.h"
+#include "src/base/platform/platform.h"
+#include "src/base/platform/time.h"
 
 namespace v8 {
 namespace platform {
@@ -53,5 +55,15 @@ void TaskQueue::Terminate() {
   process_queue_semaphore_.Signal();
 }
 
+void TaskQueue::BlockUntilQueueEmptyForTesting() {
+  for (;;) {
+    {
+      base::LockGuard<base::Mutex> guard(&lock_);
+      if (task_queue_.empty()) return;
+    }
+    base::OS::Sleep(base::TimeDelta::FromMilliseconds(5));
+  }
+}
+
 }  // namespace platform
 }  // namespace v8