Replace NULL with nullptr or null in webrtc/base/.

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2718663005
Cr-Commit-Position: refs/heads/master@{#16878}
diff --git a/webrtc/base/asyncsocket.cc b/webrtc/base/asyncsocket.cc
index 873badc..6c41596 100644
--- a/webrtc/base/asyncsocket.cc
+++ b/webrtc/base/asyncsocket.cc
@@ -19,7 +19,7 @@
 AsyncSocket::~AsyncSocket() {
 }
 
-AsyncSocketAdapter::AsyncSocketAdapter(AsyncSocket* socket) : socket_(NULL) {
+AsyncSocketAdapter::AsyncSocketAdapter(AsyncSocket* socket) : socket_(nullptr) {
   Attach(socket);
 }