Introduce ThreadTaskRunnerHandle::OverrideForTesting and TestMockTimeTaskRunner::ScopedContext.

base/test/test_simple_task_runner.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/test/test_simple_task_runner.h"
 
 #include "base/logging.h"
+#include "base/memory/ptr_util.h"
+#include "base/threading/thread_task_runner_handle.h"
 
 namespace base {
 
 TestSimpleTaskRunner::TestSimpleTaskRunner() = default;
 
 TestSimpleTaskRunner::~TestSimpleTaskRunner() = default;
 
 bool TestSimpleTaskRunner::PostDelayedTask(
     const tracked_objects::Location& from_here,
     const Closure& task,
     TimeDelta delay) {
   AutoLock auto_lock(lock_);
   pending_tasks_.push_back(
       TestPendingTask(from_here, task, TimeTicks(), delay,
                       TestPendingTask::NESTABLE));
   return true;
 }
 
 bool TestSimpleTaskRunner::PostNonNestableDelayedTask(
     const tracked_objects::Location& from_here,
     const Closure& task,
     TimeDelta delay) {
   AutoLock auto_lock(lock_);
   pending_tasks_.push_back(
       TestPendingTask(from_here, task, TimeTicks(), delay,
                       TestPendingTask::NON_NESTABLE));
   return true;
 }
 
+// TODO(gab): Use SequenceToken here to differentiate between tasks running in
+// the scope of this TestSimpleTaskRunner and other task runners sharing this
+// thread. http://crbug.com/631186
 bool TestSimpleTaskRunner::RunsTasksOnCurrentThread() const {
   return thread_ref_ == PlatformThread::CurrentRef();
 }
 
 std::deque<TestPendingTask> TestSimpleTaskRunner::TakePendingTasks() {
   AutoLock auto_lock(lock_);
   return std::move(pending_tasks_);
 }
 
 size_t TestSimpleTaskRunner::NumPendingTasks() const {
@@ skipping 24 matching lines @@
 void TestSimpleTaskRunner::RunPendingTasks() {
   DCHECK(RunsTasksOnCurrentThread());
 
   // Swap with a local variable to avoid re-entrancy problems.
   std::deque<TestPendingTask> tasks_to_run;
   {
     AutoLock auto_lock(lock_);
     tasks_to_run.swap(pending_tasks_);
   }
 
+  // Multiple test task runners can share the same thread for determinism in
+  // unit tests. Make sure this TestSimpleTaskRunner's tasks run in its scope.
+  ScopedClosureRunner undo_override;
+  if (!ThreadTaskRunnerHandle::IsSet() ||
+      ThreadTaskRunnerHandle::Get() != this) {
+    undo_override = ThreadTaskRunnerHandle::OverrideForTesting(this);
+  }
+
   for (auto& task : tasks_to_run)
     std::move(task.task).Run();
 }
 
 void TestSimpleTaskRunner::RunUntilIdle() {
   while (!pending_tasks_.empty()) {
     RunPendingTasks();
   }
 }
 
 }  // namespace base