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/bytebuffer.cc b/webrtc/base/bytebuffer.cc
index 317b6f3..859c123 100644
--- a/webrtc/base/bytebuffer.cc
+++ b/webrtc/base/bytebuffer.cc
@@ -23,12 +23,12 @@
 
 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)