[SequenceManager] Fix negative delayed_run_time_milliseconds_from_now for immediate tasks

The previous logic incorrectly dumped (delayed_run_time - now) even when
delayed_run_time was 0 (i.e. immediate tasks when
!(delayed_fence_allowed_ || add_queue_time_to_tasks), see
TaskQueueImpl::PostImmediateTaskImpl for details).

R=alexclarke@chromium.org

Bug: 1014646
Change-Id: I410b25a75b0b8703a822540e4a4e3531ded1c84e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888530
Commit-Queue: Gabriel Charette <gab@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710524}

base/task/sequence_manager/task_queue_impl.cc

@@ -910,8 +910,11 @@ void TaskQueueImpl::TaskAsValueInto(const Task& task,
   state->SetBoolean("is_cancelled", task.task.IsCancelled());
   state->SetDouble("delayed_run_time",
                    (task.delayed_run_time - TimeTicks()).InMillisecondsF());
+  const TimeDelta delayed_run_time_milliseconds_from_now =
+      task.delayed_run_time.is_null() ? TimeDelta()
+                                      : (task.delayed_run_time - now);
   state->SetDouble("delayed_run_time_milliseconds_from_now",
-                   (task.delayed_run_time - now).InMillisecondsF());
+                   delayed_run_time_milliseconds_from_now.InMillisecondsF());
   state->EndDictionary();
 }