Replace NULL with nullptr in all C++ files.

webrtc/pc/videocapturertracksource.cc

Index: webrtc/pc/videocapturertracksource.cc
diff --git a/webrtc/pc/videocapturertracksource.cc b/webrtc/pc/videocapturertracksource.cc
index 771429aecdf0ffbc4c25777bd6ca33245b36fa8e..ca08d12eadfc4e4f3ca424ad0b4434ba1c1d2246 100644
--- a/webrtc/pc/videocapturertracksource.cc
+++ b/webrtc/pc/videocapturertracksource.cc
@@ -102,7 +102,7 @@ bool NewFormatWithConstraints(
     const cricket::VideoFormat& format_in,
     bool mandatory,
     cricket::VideoFormat* format_out) {
-  RTC_DCHECK(format_out != NULL);
+  RTC_DCHECK(format_out != nullptr);
   *format_out = format_in;
 
   if (constraint.key == MediaConstraintsInterface::kMinWidth) {
@@ -261,8 +261,8 @@ rtc::scoped_refptr<VideoTrackSourceInterface> VideoCapturerTrackSource::Create(
     cricket::VideoCapturer* capturer,
     const webrtc::MediaConstraintsInterface* constraints,
     bool remote) {
-  RTC_DCHECK(worker_thread != NULL);
-  RTC_DCHECK(capturer != NULL);
+  RTC_DCHECK(worker_thread != nullptr);
+  RTC_DCHECK(capturer != nullptr);
   rtc::scoped_refptr<VideoCapturerTrackSource> source(
       new rtc::RefCountedObject<VideoCapturerTrackSource>(worker_thread,
                                                           capturer, remote));
@@ -274,8 +274,8 @@ rtc::scoped_refptr<VideoTrackSourceInterface> VideoCapturerTrackSource::Create(
     rtc::Thread* worker_thread,
     cricket::VideoCapturer* capturer,
     bool remote) {
-  RTC_DCHECK(worker_thread != NULL);
-  RTC_DCHECK(capturer != NULL);
+  RTC_DCHECK(worker_thread != nullptr);
+  RTC_DCHECK(capturer != nullptr);
   rtc::scoped_refptr<VideoCapturerTrackSource> source(
       new rtc::RefCountedObject<VideoCapturerTrackSource>(worker_thread,
                                                           capturer, remote));