Replace NULL with nullptr in webrtc/video.

webrtc/video/vie_channel.h

Index: webrtc/video/vie_channel.h
diff --git a/webrtc/video/vie_channel.h b/webrtc/video/vie_channel.h
index 93ba02863bfbdfe196ada3e7442befee75e96d37..8a9d70e58edaac1efab5d96e9779b18bdd982a77 100644
--- a/webrtc/video/vie_channel.h
+++ b/webrtc/video/vie_channel.h
@@ -201,7 +201,7 @@ class ViEChannel : public VCMFrameTypeCallback,
   template <class T>
   class RegisterableCallback : public T {
    public:
-    RegisterableCallback() : callback_(NULL) {}
+    RegisterableCallback() : callback_(nullptr) {}
 
     void Set(T* callback) {
       rtc::CritScope lock(&critsect_);