[SequenceManager] Fix NullTaskRunner::RunsTasksInCurrentSequence()

Blocks https://chromium-review.googlesource.com/c/chromium/src/+/1689339/21

R=altimin@chromium.org

Bug: 984057
Change-Id: I48f93eced8391c2b8f85bd4b27ccd3891f6b429d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724689
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681890}

base/task/sequence_manager/task_queue.cc

@@ -12,6 +12,7 @@
 #include "base/task/sequence_manager/sequence_manager_impl.h"
 #include "base/task/sequence_manager/task_queue_impl.h"
 #include "base/threading/thread_checker.h"
+#include "base/threading/thread_checker_impl.h"
 #include "base/time/time.h"
 
 namespace base {
@@ -35,11 +36,15 @@ class NullTaskRunner final : public SingleThreadTaskRunner {
     return false;
   }
 
-  bool RunsTasksInCurrentSequence() const override { return false; }
+  bool RunsTasksInCurrentSequence() const override {
+    return thread_checker_.CalledOnValidThread();
+  }
 
  private:
   // Ref-counted
   ~NullTaskRunner() override = default;
+
+  ThreadCheckerImpl thread_checker_;
 };
 
 // TODO(kraynov): Move NullTaskRunner from //base/test to //base.