Introduce ThreadTaskRunnerHandle::OverrideForTesting and TestMockTimeTaskRunner::ScopedContext.

base/threading/thread_task_runner_handle.h

Index: base/threading/thread_task_runner_handle.h
diff --git a/base/threading/thread_task_runner_handle.h b/base/threading/thread_task_runner_handle.h
index c8e58935f0977259349857bf91d71c63bf734567..8baf4544c9ed31e83ef2870c3364603d5f79b506 100644
--- a/base/threading/thread_task_runner_handle.h
+++ b/base/threading/thread_task_runner_handle.h
@@ -32,8 +32,30 @@ class BASE_EXPORT ThreadTaskRunnerHandle {
       scoped_refptr<SingleThreadTaskRunner> task_runner);
   ~ThreadTaskRunnerHandle();
 
+  // A ThreadTaskRunnerHandle::NestedForTesting is the same thing as a
+  // ThreadTaskRunnerHandle except that it allows overriding an existing
+  // ThreadTaskRunnerHandle in its scope. Nesting ThreadTaskRunnerHandles isn't
+  // generally desired but it can be useful in unit tests where multiple task
+  // runners are sharing the main thread for simplicity and determinism.
+  class BASE_EXPORT NestedForTesting {
+   public:
+    explicit NestedForTesting(
+        scoped_refptr<SingleThreadTaskRunner> task_runner);
+    ~NestedForTesting();
+
+   private:
+    friend class ThreadTaskRunnerHandle;
+
+    scoped_refptr<SingleThreadTaskRunner> task_runner_;
+    const NestedForTesting* const previous_handle_;
+
+    DISALLOW_COPY_AND_ASSIGN(NestedForTesting);
+  };
+
  private:
-  scoped_refptr<SingleThreadTaskRunner> task_runner_;
+  // The impl uses NestedForTesting to avoid code duplication but this is the
+  // only allowed non-test usage.
+  std::unique_ptr<NestedForTesting> handle_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadTaskRunnerHandle);
 };