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

net/quic/test_tools/mock_clock.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_QUIC_TEST_TOOLS_MOCK_CLOCK_H_
 #define NET_QUIC_TEST_TOOLS_MOCK_CLOCK_H_
 
 #include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "net/quic/platform/api/quic_clock.h"
 
 namespace net {
 
 class MockClock : public QuicClock {
  public:
   MockClock();
   ~MockClock() override;
 
+  // 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);
 
-  QuicTime Now() const override;
-
-  QuicTime ApproximateNow() const override;
-
-  QuicWallTime WallNow() const override;
-
+  // Returns the current time in ticks.
   base::TimeTicks NowInTicks() const;
 
  private:
   QuicTime now_;
 
   DISALLOW_COPY_AND_ASSIGN(MockClock);
 };
 
 }  // namespace net
 
 #endif  // NET_QUIC_TEST_TOOLS_MOCK_CLOCK_H_