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

webrtc/base/fakeclock.h

Index: webrtc/base/fakeclock.h
diff --git a/webrtc/base/fakeclock.h b/webrtc/base/fakeclock.h
index 2b3afdde05f684926621bcc95c2f917bfb7bd60f..4aecc54cc4ffee3e894cd4fef97760fce69338e8 100644
--- a/webrtc/base/fakeclock.h
+++ b/webrtc/base/fakeclock.h
@@ -19,6 +19,8 @@ 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 {}
@@ -38,6 +40,17 @@ class FakeClock : public ClockInterface {
   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_