Adds fake clock unit test.

Also removing unused ScopedRealClock.

Bug: webrtc:10365
Change-Id: I3cd95df894cba9bf4ee703d979f476e2c526bf35
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128860
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27425}

rtc_base/BUILD.gn

@@ -1359,6 +1359,7 @@ if (rtc_include_tests) {
       "callback_unittest.cc",
       "crc32_unittest.cc",
       "data_rate_limiter_unittest.cc",
+      "fake_clock_unittest.cc",
       "helpers_unittest.cc",
       "ip_address_unittest.cc",
       "memory_usage_unittest.cc",

rtc_base/fake_clock.h

@@ -62,16 +62,6 @@ class ScopedFakeClock : public FakeClock {
   ClockInterface* prev_clock_;
 };
 
-// Helper class to "undo" the fake clock temporarily.
-class ScopedRealClock {
- public:
-  ScopedRealClock();
-  ~ScopedRealClock();
-
- private:
-  ClockInterface* prev_clock_;
-};
-
 }  // namespace rtc
 
 #endif  // RTC_BASE_FAKE_CLOCK_H_

rtc_base/fake_clock_unittest.cc

@@ -0,0 +1,37 @@
+/*
+ *  Copyright 2019 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/gtest.h"
+
+#include "rtc_base/fake_clock.h"
+
+namespace rtc {
+TEST(ScopedFakeClockTest, OverridesGlobalClock) {
+  const int64_t kFixedTimeUs = 100000;
+  int64_t real_time_us = rtc::TimeMicros();
+  EXPECT_NE(real_time_us, 0);
+  {
+    ScopedFakeClock scoped;
+    EXPECT_EQ(rtc::TimeMicros(), 0);
+
+    scoped.AdvanceTimeMicros(1000);
+    EXPECT_EQ(rtc::TimeMicros(), 1000);
+
+    scoped.SetTimeMicros(kFixedTimeUs);
+    EXPECT_EQ(rtc::TimeMicros(), kFixedTimeUs);
+
+    scoped.AdvanceTimeMicros(1000);
+    EXPECT_EQ(rtc::TimeMicros(), kFixedTimeUs + 1000);
+  }
+
+  EXPECT_NE(rtc::TimeMicros(), kFixedTimeUs + 1000);
+  EXPECT_GE(rtc::TimeMicros(), real_time_us);
+}
+}  // namespace rtc