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

components/sync/base/cancelation_signal.h

Index: components/sync/base/cancelation_signal.h
diff --git a/components/sync/base/cancelation_signal.h b/components/sync/base/cancelation_signal.h
index 6740743f8e129d6facc3e3960bcf245439c0e6f1..0bf72e7f8784e401693ae1ddab642cae32b2019d 100644
--- a/components/sync/base/cancelation_signal.h
+++ b/components/sync/base/cancelation_signal.h
@@ -62,7 +62,7 @@ class CancelationSignal {
   // True if Signal() has been invoked.
   bool signalled_;
 
-  // The registered abort handler.  May be NULL.
+  // The registered abort handler.  May be null.
   CancelationObserver* handler_;
 };