Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/clock.cc

 /*
  *  Copyright (c) 2013 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 208 matching lines @@
       nullptr) {
     // g_shared_clock was assigned while we constructed/tried to assign our
     // instance, delete our instance and use the existing one.
     delete clock;
   }
   return g_shared_clock;
 #elif defined(WEBRTC_LINUX) || defined(WEBRTC_MAC)
   static UnixRealTimeClock clock;
   return &clock;
 #else
-  return NULL;
+  return nullptr;
 #endif
 }
 
 SimulatedClock::SimulatedClock(int64_t initial_time_us)
     : time_us_(initial_time_us), lock_(RWLockWrapper::CreateRWLock()) {
 }
 
 SimulatedClock::~SimulatedClock() {
 }
 
@@ skipping 21 matching lines @@
 void SimulatedClock::AdvanceTimeMilliseconds(int64_t milliseconds) {
   AdvanceTimeMicroseconds(1000 * milliseconds);
 }
 
 void SimulatedClock::AdvanceTimeMicroseconds(int64_t microseconds) {
   WriteLockScoped synchronize(*lock_);
   time_us_ += microseconds;
 }
 
 };  // namespace webrtc