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

components/sync/model/syncable_service.h

Index: components/sync/model/syncable_service.h
diff --git a/components/sync/model/syncable_service.h b/components/sync/model/syncable_service.h
index ddde733ab9b3a8d114293318a85fc2fd6d6a9977..dcd637cbbf7f28fa2c81e31981fbd9c108a6d21c 100644
--- a/components/sync/model/syncable_service.h
+++ b/components/sync/model/syncable_service.h
@@ -67,9 +67,9 @@ class SyncableService : public SyncChangeProcessor,
   // Returns AttachmentStore for use by sync when uploading or downloading
   // attachments.
   // GetAttachmentStoreForSync is called right before MergeDataAndStartSyncing.
-  // If at that time GetAttachmentStoreForSync returns NULL then datatype is
+  // If at that time GetAttachmentStoreForSync returns null then datatype is
   // considered not using attachments and all attempts to upload/download
-  // attachments will fail. Default implementation returns NULL. Datatype that
+  // attachments will fail. Default implementation returns null. Datatype that
   // uses sync attachments should create attachment store, implement
   // GetAttachmentStoreForSync to return result of
   // AttachmentStore::CreateAttachmentStoreForSync() from attachment store
@@ -80,7 +80,7 @@ class SyncableService : public SyncChangeProcessor,
   // attachments.
   // SetAttachmentService is called after GetAttachmentStore and right before
   // MergeDataAndStartSyncing and only if GetAttachmentStore has returned a
-  // non-NULL store instance. Default implementation does nothing.
+  // non-null store instance. Default implementation does nothing.
   // Datatype that uses attachments must take ownerhip of the provided
   // AttachmentService instance.
   virtual void SetAttachmentService(