[Sync] Replacing NULL with nullptr/null throughout sync code.

components/sync_sessions/session_data_type_controller_unittest.cc

Index: components/sync_sessions/session_data_type_controller_unittest.cc
diff --git a/components/sync_sessions/session_data_type_controller_unittest.cc b/components/sync_sessions/session_data_type_controller_unittest.cc
index d69704d5ebc541f5faf49dc64e58a2d4c4362c33..8e4b7b703e6ee0cf349683b08ac7dcb3a693309a 100644
--- a/components/sync_sessions/session_data_type_controller_unittest.cc
+++ b/components/sync_sessions/session_data_type_controller_unittest.cc
@@ -47,7 +47,7 @@ class MockSyncedWindowDelegate : public SyncedWindowDelegate {
   bool IsTabPinned(const SyncedTabDelegate* tab) const override {
     return false;
   }
-  SyncedTabDelegate* GetTabAt(int index) const override { return NULL; }
+  SyncedTabDelegate* GetTabAt(int index) const override { return nullptr; }
   SessionID::id_type GetTabIdAt(int index) const override { return 0; }
 
   bool IsSessionRestoreInProgress() const override {