Replace NULL with nullptr in all C++ files.

webrtc/pc/datachannel.cc

Index: webrtc/pc/datachannel.cc
diff --git a/webrtc/pc/datachannel.cc b/webrtc/pc/datachannel.cc
index 5606a64c6e2b244b9d16954cdf0c6b4e2038c70a..05be423e5dab9c39bc800052ab07bbbb79eae648 100644
--- a/webrtc/pc/datachannel.cc
+++ b/webrtc/pc/datachannel.cc
@@ -117,7 +117,7 @@ rtc::scoped_refptr<DataChannel> DataChannel::Create(
   rtc::scoped_refptr<DataChannel> channel(
       new rtc::RefCountedObject<DataChannel>(provider, dct, label));
   if (!channel->Init(config)) {
-    return NULL;
+    return nullptr;
   }
   return channel;
 }
@@ -192,7 +192,8 @@ bool DataChannel::Init(const InternalDataChannelInit& config) {
     // Chrome glue and WebKit) are not wired up properly until after this
     // function returns.
     if (provider_->ReadyToSendData()) {
-      rtc::Thread::Current()->Post(RTC_FROM_HERE, this, MSG_CHANNELREADY, NULL);
+      rtc::Thread::Current()->Post(RTC_FROM_HERE, this, MSG_CHANNELREADY,
+                                   nullptr);
     }
   }
 
@@ -207,7 +208,7 @@ void DataChannel::RegisterObserver(DataChannelObserver* observer) {
 }
 
 void DataChannel::UnregisterObserver() {
-  observer_ = NULL;
+  observer_ = nullptr;
 }
 
 bool DataChannel::reliable() const {