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..da41d8dec4a7e65755004fda0f195d712ccafcfc 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 +40,20 @@ class FakeClock : public ClockInterface {
   uint64_t time_ GUARDED_BY(lock_) = 0u;
 };
 
+// Helper class which creates and sets a fake clock as the global clock in
+// its constructor, and unsets and deletes it in its destructor.
+class ScopedFakeClock {
+ public:
+  ScopedFakeClock() { SetClock(&clock_); }
+
+  ~ScopedFakeClock() { SetClock(nullptr); }
+
+  FakeClock& clock() { return clock_; }
+
+ private:
+  FakeClock clock_;
+};
+
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_FAKECLOCK_H_