Introduce ThreadTaskRunnerHandle::OverrideForTesting and TestMockTimeTaskRunner::ScopedContext.

base/threading/thread_task_runner_handle.cc

Index: base/threading/thread_task_runner_handle.cc
diff --git a/base/threading/thread_task_runner_handle.cc b/base/threading/thread_task_runner_handle.cc
index 190e18ffc68557c64f81f5b766a952c37d3c34a7..ab2243a646116153b48464365b904b5dab7879b5 100644
--- a/base/threading/thread_task_runner_handle.cc
+++ b/base/threading/thread_task_runner_handle.cc
@@ -8,6 +8,7 @@
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
+#include "base/memory/ptr_util.h"
 #include "base/threading/sequenced_task_runner_handle.h"
 #include "base/threading/thread_local.h"
 
@@ -15,14 +16,16 @@ namespace base {
 
 namespace {
 
-base::LazyInstance<base::ThreadLocalPointer<ThreadTaskRunnerHandle>>::Leaky
-    lazy_tls_ptr = LAZY_INSTANCE_INITIALIZER;
+base::LazyInstance<base::ThreadLocalPointer<
+    const ThreadTaskRunnerHandle::NestedForTesting>>::Leaky lazy_tls_ptr =
+    LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
 // static
 scoped_refptr<SingleThreadTaskRunner> ThreadTaskRunnerHandle::Get() {
-  ThreadTaskRunnerHandle* current = lazy_tls_ptr.Pointer()->Get();
+  const ThreadTaskRunnerHandle::NestedForTesting* current =
+      lazy_tls_ptr.Pointer()->Get();
   DCHECK(current);
   return current->task_runner_;
 }
@@ -33,19 +36,31 @@ bool ThreadTaskRunnerHandle::IsSet() {
 }
 
 ThreadTaskRunnerHandle::ThreadTaskRunnerHandle(
-    scoped_refptr<SingleThreadTaskRunner> task_runner)
-    : task_runner_(std::move(task_runner)) {
-  DCHECK(task_runner_->BelongsToCurrentThread());
+    scoped_refptr<SingleThreadTaskRunner> task_runner) {
   // No SequencedTaskRunnerHandle (which includes ThreadTaskRunnerHandles)
-  // should already be set for this thread.
+  // should already be set for this thread. Nesting isn't allowed outside of
+  // tests (ref. NestedForTesting).
   DCHECK(!SequencedTaskRunnerHandle::IsSet());
+
+  // After checking that nesting wasn't used outside of tests above, use
+  // NestedForTesting for the assignement logic.
+  handle_ = MakeUnique<NestedForTesting>(std::move(task_runner));
+}
+
+ThreadTaskRunnerHandle::~ThreadTaskRunnerHandle() = default;
+
+ThreadTaskRunnerHandle::NestedForTesting::NestedForTesting(
+    scoped_refptr<SingleThreadTaskRunner> task_runner)
+    : task_runner_(std::move(task_runner)),
+      previous_handle_(lazy_tls_ptr.Pointer()->Get()) {
+  DCHECK(task_runner_->BelongsToCurrentThread());
   lazy_tls_ptr.Pointer()->Set(this);
 }
 
-ThreadTaskRunnerHandle::~ThreadTaskRunnerHandle() {
+ThreadTaskRunnerHandle::NestedForTesting::~NestedForTesting() {
   DCHECK(task_runner_->BelongsToCurrentThread());
   DCHECK_EQ(lazy_tls_ptr.Pointer()->Get(), this);
-  lazy_tls_ptr.Pointer()->Set(nullptr);
+  lazy_tls_ptr.Pointer()->Set(previous_handle_);
 }
 
 }  // namespace base