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

webrtc/base/bytebuffer.cc

Index: webrtc/base/bytebuffer.cc
diff --git a/webrtc/base/bytebuffer.cc b/webrtc/base/bytebuffer.cc
index 317b6f383a564c904a1ea1d891b5cf90a38f2b6b..859c123f025e9d4fda52bada23807c55455b8577 100644
--- a/webrtc/base/bytebuffer.cc
+++ b/webrtc/base/bytebuffer.cc
@@ -23,12 +23,12 @@ static const int DEFAULT_SIZE = 4096;
 
 ByteBufferWriter::ByteBufferWriter()
     : ByteBuffer(ORDER_NETWORK) {
-  Construct(NULL, DEFAULT_SIZE);
+  Construct(nullptr, DEFAULT_SIZE);
 }
 
 ByteBufferWriter::ByteBufferWriter(ByteOrder byte_order)
     : ByteBuffer(byte_order) {
-  Construct(NULL, DEFAULT_SIZE);
+  Construct(nullptr, DEFAULT_SIZE);
 }
 
 ByteBufferWriter::ByteBufferWriter(const char* bytes, size_t len)