Introduce ThreadTaskRunnerHandle::OverrideForTesting and TestMockTimeTaskRunner::ScopedContext.

base/threading/thread_task_runner_handle.h

 // Copyright 2016 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.
 
 #ifndef BASE_THREADING_THREAD_TASK_RUNNER_HANDLE_H_
 #define BASE_THREADING_THREAD_TASK_RUNNER_HANDLE_H_
 
 #include "base/base_export.h"
+#include "base/callback_helpers.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 
 namespace base {
 
 // ThreadTaskRunnerHandle stores a reference to a thread's TaskRunner
 // in thread-local storage.  Callers can then retrieve the TaskRunner
 // for the current thread by calling ThreadTaskRunnerHandle::Get().
 // At most one TaskRunner may be bound to each thread at a time.
 // Prefer SequenceTaskRunnerHandle to this unless thread affinity is required.
 class BASE_EXPORT ThreadTaskRunnerHandle {
  public:
   // Gets the SingleThreadTaskRunner for the current thread.
   static scoped_refptr<SingleThreadTaskRunner> Get();
 
   // Returns true if the SingleThreadTaskRunner is already created for
   // the current thread.
   static bool IsSet();
 
+  // Overrides ThreadTaskRunnerHandle::Get()'s |task_runner_| to point at
+  // |overriding_task_runner| until the returned ScopedClosureRunner goes out of
+  // scope (instantiates a ThreadTaskRunnerHandle for that scope if |!IsSet()|).
+  // Nested overrides are allowed but callers must ensure the
+  // ScopedClosureRunners expire in LIFO (stack) order. Note: nesting
+  // ThreadTaskRunnerHandles isn't generally desired but it's useful in unit
+  // tests where multiple task runners can share the main thread for simplicity
+  // and determinism.
+  static ScopedClosureRunner OverrideForTesting(
+      scoped_refptr<SingleThreadTaskRunner> overriding_task_runner);
+
   // Binds |task_runner| to the current thread. |task_runner| must belong
   // to the current thread for this to succeed.
   explicit ThreadTaskRunnerHandle(
       scoped_refptr<SingleThreadTaskRunner> task_runner);
   ~ThreadTaskRunnerHandle();
 
  private:
   scoped_refptr<SingleThreadTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadTaskRunnerHandle);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_TASK_RUNNER_HANDLE_H_