[Open Screen] Implement IsRunningOnTaskRunner().

This API was not implemented in the Chrome task runner.
The implementation will allow us to make the API pure virtual upstream.

Change-Id: I44598d5c0410ed6019de7c8042bfa2ae54b96544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040112Reviewed-by: Brandon Tolsch <btolsch@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739019}

components/openscreen_platform/platform_task_runner.cc

@@ -44,4 +44,8 @@ void PlatformTaskRunner::PostPackagedTaskWithDelay(TaskRunner::Task task,
       FROM_HERE, base::BindOnce(ExecuteTask, std::move(task)), time_delta);
 }
 
+bool PlatformTaskRunner::IsRunningOnTaskRunner() {
+  return task_runner_->RunsTasksInCurrentSequence();
+}
+
 }  // namespace openscreen_platform

components/openscreen_platform/platform_task_runner.h

@@ -26,6 +26,7 @@ class PlatformTaskRunner final : public openscreen::TaskRunner {
   void PostPackagedTask(openscreen::TaskRunner::Task task) final;
   void PostPackagedTaskWithDelay(openscreen::TaskRunner::Task task,
                                  openscreen::Clock::duration delay) final;
+  bool IsRunningOnTaskRunner() final;
 
  private:
   scoped_refptr<base::SequencedTaskRunner> task_runner_;