Add ScopedTaskEnvironment::GetMainThreadTaskRunner().

base/test/scoped_task_environment.cc

Index: base/test/scoped_task_environment.cc
diff --git a/base/test/scoped_task_environment.cc b/base/test/scoped_task_environment.cc
index 22ed28cd2c6f535b6049195583a1f7d1327ae3d3..8ccf040a3fea88d3ca22114d07c5773a468cf001 100644
--- a/base/test/scoped_task_environment.cc
+++ b/base/test/scoped_task_environment.cc
@@ -66,6 +66,11 @@ ScopedTaskEnvironment::~ScopedTaskEnvironment() {
   TaskScheduler::SetInstance(nullptr);
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+ScopedTaskEnvironment::GetMainThreadTaskRunner() {
+  return message_loop_.task_runner();
+}
+
 void ScopedTaskEnvironment::RunUntilIdle() {
   for (;;) {
     TaskScheduler::GetInstance()->FlushForTesting();