[ThreadPool]: Add JobHandle bool conversion.

Bug: 839091
Change-Id: Ie40872cdd6a7ea1d5d628a8aa49b0df5b9f48a3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904269
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713904}

base/task/post_job.h

@@ -84,6 +84,9 @@ class BASE_EXPORT JobHandle {
   JobHandle(JobHandle&&);
   JobHandle& operator=(JobHandle&&);
 
+  // Returns true if associated with a Job.
+  explicit operator bool() const { return task_source_ != nullptr; }
+
   // Update this Job's priority.
   void UpdatePriority(TaskPriority new_priority);