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

components/sync/engine_impl/cycle/directory_type_debug_info_emitter.cc

Index: components/sync/engine_impl/cycle/directory_type_debug_info_emitter.cc
diff --git a/components/sync/engine_impl/cycle/directory_type_debug_info_emitter.cc b/components/sync/engine_impl/cycle/directory_type_debug_info_emitter.cc
index 975fd463755bbe97d447e2cbd17c916210cf8e03..a7f6f357ae286f5ebdc98559f14bea2540ed451b 100644
--- a/components/sync/engine_impl/cycle/directory_type_debug_info_emitter.cc
+++ b/components/sync/engine_impl/cycle/directory_type_debug_info_emitter.cc
@@ -26,7 +26,7 @@ DirectoryTypeDebugInfoEmitter::DirectoryTypeDebugInfoEmitter(
 DirectoryTypeDebugInfoEmitter::DirectoryTypeDebugInfoEmitter(
     ModelType type,
     base::ObserverList<TypeDebugInfoObserver>* observers)
-    : directory_(NULL), type_(type), type_debug_info_observers_(observers) {}
+    : directory_(nullptr), type_(type), type_debug_info_observers_(observers) {}
 
 DirectoryTypeDebugInfoEmitter::~DirectoryTypeDebugInfoEmitter() {}