Replace NULL with nullptr in all C++ files.

webrtc/pc/channelmanager.cc

Index: webrtc/pc/channelmanager.cc
diff --git a/webrtc/pc/channelmanager.cc b/webrtc/pc/channelmanager.cc
index d0957a7b55f507af873a4b8b554c78865977408f..f3db307960956ffd9887d5f5201375fe9d2db2f6 100644
--- a/webrtc/pc/channelmanager.cc
+++ b/webrtc/pc/channelmanager.cc
@@ -190,7 +190,7 @@ void ChannelManager::Terminate() {
 
 void ChannelManager::DestructorDeletes_w() {
   RTC_DCHECK(worker_thread_ == rtc::Thread::Current());
-  media_engine_.reset(NULL);
+  media_engine_.reset(nullptr);
 }
 
 void ChannelManager::Terminate_w() {
@@ -309,8 +309,8 @@ VideoChannel* ChannelManager::CreateVideoChannel_w(
   RTC_DCHECK(nullptr != media_controller);
   VideoMediaChannel* media_channel = media_engine_->CreateVideoChannel(
       media_controller->call_w(), media_controller->config(), options);
-  if (media_channel == NULL) {
-    return NULL;
+  if (media_channel == nullptr) {
+    return nullptr;
   }
 
   VideoChannel* video_channel = new VideoChannel(
@@ -320,7 +320,7 @@ VideoChannel* ChannelManager::CreateVideoChannel_w(
   if (!video_channel->Init_w(rtp_transport, rtcp_transport, rtp_transport,
                              rtcp_transport)) {
     delete video_channel;
-    return NULL;
+    return nullptr;
   }
   video_channels_.push_back(video_channel);
   return video_channel;