[Sync] Replacing NULL with nullptr in code and null in comments for components/sync/

components/sync/syncable/read_node.cc

Index: components/sync/syncable/read_node.cc
diff --git a/components/sync/syncable/read_node.cc b/components/sync/syncable/read_node.cc
index 2cc1c867a00d2e5c95514688b51a5552e96f8e0d..436ed7ee3199b50d3fe41afc40a274cca8278eb7 100644
--- a/components/sync/syncable/read_node.cc
+++ b/components/sync/syncable/read_node.cc
@@ -15,13 +15,13 @@ namespace syncer {
 //////////////////////////////////////////////////////////////////////////
 // ReadNode member definitions
 ReadNode::ReadNode(const BaseTransaction* transaction)
-    : entry_(NULL), transaction_(transaction) {
+    : entry_(nullptr), transaction_(transaction) {
   DCHECK(transaction);
 }
 
 ReadNode::ReadNode() {
-  entry_ = NULL;
-  transaction_ = NULL;
+  entry_ = nullptr;
+  transaction_ = nullptr;
 }
 
 ReadNode::~ReadNode() {