Introduce a RunLoop class that supports the TaskQueue interface
on the current thread.

This simplifies writing async tests that use TaskQueue and doesn't
require spinning up a new thread for simple things. The implementation
is currently based on rtc::Thread, which could also be useful in
some circumstances while migrating code over to TQ.

Remove PressEnterToContinue from the test_common files since
it's very specific and only used from one file.

Bug: none
Change-Id: I8b2c6c40809271a109ec17cf7e1120847645d58a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174260
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31160}
diff --git a/test/run_loop_unittest.cc b/test/run_loop_unittest.cc
new file mode 100644
index 0000000..a356cc2
--- /dev/null
+++ b/test/run_loop_unittest.cc
@@ -0,0 +1,61 @@
+/*
+ *  Copyright (c) 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "test/run_loop.h"
+
+#include "rtc_base/task_queue.h"
+#include "rtc_base/task_utils/to_queued_task.h"
+#include "test/gtest.h"
+
+namespace webrtc {
+
+TEST(RunLoopTest, TaskQueueOnThread) {
+  EXPECT_EQ(TaskQueueBase::Current(), nullptr);
+  test::RunLoop loop;
+  EXPECT_EQ(TaskQueueBase::Current(), loop.task_queue());
+  EXPECT_TRUE(loop.task_queue()->IsCurrent());
+}
+
+TEST(RunLoopTest, Flush) {
+  test::RunLoop loop;
+  int counter = 0;
+  loop.PostTask([&counter]() { ++counter; });
+  EXPECT_EQ(counter, 0);
+  loop.Flush();
+  EXPECT_EQ(counter, 1);
+}
+
+TEST(RunLoopTest, Delayed) {
+  test::RunLoop loop;
+  bool ran = false;
+  loop.PostDelayedTask(
+      [&ran, &loop]() {
+        ran = true;
+        loop.Quit();
+      },
+      100);
+  loop.Flush();
+  EXPECT_FALSE(ran);
+  loop.Run();
+  EXPECT_TRUE(ran);
+}
+
+TEST(RunLoopTest, PostAndQuit) {
+  test::RunLoop loop;
+  bool ran = false;
+  loop.PostTask([&ran, &loop]() {
+    ran = true;
+    loop.Quit();
+  });
+  loop.Run();
+  EXPECT_TRUE(ran);
+}
+
+}  // namespace webrtc