Remove non-monotonic clock support

webrtc/system_wrappers/source/tick_util.cc

 /*
  *  Copyright (c) 2012 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.
  */
 
@@ skipping 88 matching lines @@
     // 0x0fffffff ~3.1 days, the code will not take that long to execute
     // so it must have been a wrap around.
     if (old > 0xf0000000 && now < 0x0fffffff) {
       // TODO(pbos): Fix unsafe use of static locals.
       num_wrap_time_get_time++;
     }
   }
   return now + (num_wrap_time_get_time << 32);
 #elif defined(WEBRTC_LINUX)
   struct timespec ts;
-  // TODO(wu): Remove CLOCK_REALTIME implementation.
-#ifdef WEBRTC_CLOCK_TYPE_REALTIME
-  clock_gettime(CLOCK_REALTIME, &ts);
-#else
   clock_gettime(CLOCK_MONOTONIC, &ts);
-#endif
   return 1000000000LL * ts.tv_sec + ts.tv_nsec;
 #elif defined(WEBRTC_MAC)
   // Return absolute time as an offset from the first call to this function, so
   // that we can do floating-point (double) operations on it without losing
   // precision. This holds true until the elapsed time is ~11 days,
   // at which point we'll start to lose some precision, though not enough to
   // matter for millisecond accuracy for another couple years after that.
   // TODO(pbos): Fix unsafe use of static locals.
   static uint64_t timebase_start = 0;
   if (timebase_start == 0) {
     timebase_start = mach_absolute_time();
   }
   return mach_absolute_time() - timebase_start;
 #else
   struct timeval tv;
   gettimeofday(&tv, NULL);
   return 1000000LL * tv.tv_sec + tv.tv_usec;
 #endif
 }
 
 }  // namespace webrtc