Fix deadlock in VideoSendStream tests, cause of flake on some bots.

Bug: webrtc:10861, webrtc:10880
Change-Id: Ic3ff9fab420e1fd634f58ef86d2f8890e23cfd03
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150220
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Yves Gerey <yvesg@google.com>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28969}

test/single_threaded_task_queue.h

@@ -32,6 +32,7 @@ class SingleThreadedTaskQueueForTesting {
  public:
   using Task = std::function<void()>;
   using TaskId = size_t;
+  constexpr static TaskId kInvalidTaskId = static_cast<TaskId>(-1);
 
   explicit SingleThreadedTaskQueueForTesting(const char* name);
   ~SingleThreadedTaskQueueForTesting();
@@ -59,6 +60,13 @@ class SingleThreadedTaskQueueForTesting {
   // Returns true iff called on the thread associated with the task queue.
   bool IsCurrent();
 
+  // Returns true iff the task queue is actively being serviced.
+  bool IsRunning();
+
+  bool HasPendingTasks() const;
+
+  void Stop();
+
  private:
   struct QueuedTask {
     QueuedTask(TaskId task_id, int64_t earliest_execution_time, Task task);