Improving the fake clock and using it to fix a flaky STUN timeout test.

webrtc/base/fakeclock.h

 /*
  *  Copyright 2016 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.
  */
 
 #ifndef WEBRTC_BASE_FAKECLOCK_H_
 #define WEBRTC_BASE_FAKECLOCK_H_
 
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/timedelta.h"
 #include "webrtc/base/timeutils.h"
 
 namespace rtc {
 
 // Fake clock for use with unit tests, which does not tick on its own.
 // Starts at time 0.
+//
+// TODO(deadbeef): Unify with webrtc::SimulatedClock.
 class FakeClock : public ClockInterface {
  public:
   ~FakeClock() override {}
 
   // ClockInterface implementation.
   uint64_t TimeNanos() const override;
 
   // Methods that can be used by the test to control the time.
 
   // Should only be used to set a time in the future.
   void SetTimeNanos(uint64_t nanos);
 
   void AdvanceTime(TimeDelta delta);
 
  private:
   CriticalSection lock_;
   uint64_t time_ GUARDED_BY(lock_) = 0u;
 };
 
+// Helper class that sets itself as the global clock in its constructor and
+// unsets it in its destructor.
+class ScopedFakeClock : public FakeClock {
+ public:
+  ScopedFakeClock();
+  ~ScopedFakeClock() override;
+
+ private:
+  ClockInterface* prev_clock_;
+};
+
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_FAKECLOCK_H_