Replace NULL with nullptr in all C++ files.

webrtc/modules/rtp_rtcp/source/receive_statistics_impl.cc

Index: webrtc/modules/rtp_rtcp/source/receive_statistics_impl.cc
diff --git a/webrtc/modules/rtp_rtcp/source/receive_statistics_impl.cc b/webrtc/modules/rtp_rtcp/source/receive_statistics_impl.cc
index 6d64304149d662f746c78563e389a1e03deea921..91c9225e53b875c6860befbbb69e92d65d03e372 100644
--- a/webrtc/modules/rtp_rtcp/source/receive_statistics_impl.cc
+++ b/webrtc/modules/rtp_rtcp/source/receive_statistics_impl.cc
@@ -380,8 +380,8 @@ ReceiveStatistics* ReceiveStatistics::Create(Clock* clock) {
 
 ReceiveStatisticsImpl::ReceiveStatisticsImpl(Clock* clock)
     : clock_(clock),
-      rtcp_stats_callback_(NULL),
-      rtp_stats_callback_(NULL) {}
+      rtcp_stats_callback_(nullptr),
+      rtp_stats_callback_(nullptr) {}
 
 ReceiveStatisticsImpl::~ReceiveStatisticsImpl() {
   while (!statisticians_.empty()) {
@@ -442,7 +442,7 @@ StreamStatistician* ReceiveStatisticsImpl::GetStatistician(
   rtc::CritScope cs(&receive_statistics_lock_);
   StatisticianImplMap::const_iterator it = statisticians_.find(ssrc);
   if (it == statisticians_.end())
-    return NULL;
+    return nullptr;
   return it->second;
 }
 
@@ -458,8 +458,8 @@ void ReceiveStatisticsImpl::SetMaxReorderingThreshold(
 void ReceiveStatisticsImpl::RegisterRtcpStatisticsCallback(
     RtcpStatisticsCallback* callback) {
   rtc::CritScope cs(&receive_statistics_lock_);
-  if (callback != NULL)
-    assert(rtcp_stats_callback_ == NULL);
+  if (callback != nullptr)
+    assert(rtcp_stats_callback_ == nullptr);
   rtcp_stats_callback_ = callback;
 }
 
@@ -479,8 +479,8 @@ void ReceiveStatisticsImpl::CNameChanged(const char* cname, uint32_t ssrc) {
 void ReceiveStatisticsImpl::RegisterRtpStatisticsCallback(
     StreamDataCountersCallback* callback) {
   rtc::CritScope cs(&receive_statistics_lock_);
-  if (callback != NULL)
-    assert(rtp_stats_callback_ == NULL);
+  if (callback != nullptr)
+    assert(rtp_stats_callback_ == nullptr);
   rtp_stats_callback_ = callback;
 }
 
@@ -505,7 +505,7 @@ StatisticianMap NullReceiveStatistics::GetActiveStatisticians() const {
 
 StreamStatistician* NullReceiveStatistics::GetStatistician(
     uint32_t ssrc) const {
-  return NULL;
+  return nullptr;
 }
 
 void NullReceiveStatistics::SetMaxReorderingThreshold(