Add a QuicChromiumClock and make QuicClock abstract to match internal code.

net/quic/test_tools/mock_clock.h

Index: net/quic/test_tools/mock_clock.h
diff --git a/net/quic/test_tools/mock_clock.h b/net/quic/test_tools/mock_clock.h
index a8bfc731b1eacd70c1fd3f545fa1dff57e899025..5bcdf0b9ce23e20ca627f1219ebc66ad71a692d0 100644
--- a/net/quic/test_tools/mock_clock.h
+++ b/net/quic/test_tools/mock_clock.h
@@ -18,14 +18,15 @@ class MockClock : public QuicClock {
   MockClock();
   ~MockClock() override;
 
-  void AdvanceTime(QuicTime::Delta delta);
-
+  // QuicClock implementation:
   QuicTime Now() const override;
-
   QuicTime ApproximateNow() const override;
-
   QuicWallTime WallNow() const override;
 
+  // Advances the current time by |delta|, which may be negative.
+  void AdvanceTime(QuicTime::Delta delta);
+
+  // Returns the current time in ticks.
   base::TimeTicks NowInTicks() const;
 
  private: