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

components/sync/test/engine/mock_connection_manager.cc

Index: components/sync/test/engine/mock_connection_manager.cc
diff --git a/components/sync/test/engine/mock_connection_manager.cc b/components/sync/test/engine/mock_connection_manager.cc
index c9cb07476b3589af6223577560ded52b0b7aea99..46123ac4d5ff7d162f26b059b94095dce59a2e7f 100644
--- a/components/sync/test/engine/mock_connection_manager.cc
+++ b/components/sync/test/engine/mock_connection_manager.cc
@@ -45,7 +45,7 @@ MockConnectionManager::MockConnectionManager(syncable::Directory* directory,
       client_stuck_(false),
       countdown_to_postbuffer_fail_(0),
       directory_(directory),
-      mid_commit_observer_(NULL),
+      mid_commit_observer_(nullptr),
       throttling_(false),
       partialThrottling_(false),
       fail_non_periodic_get_updates_(false),
@@ -739,7 +739,7 @@ MockConnectionManager::GetProgressMarkerForType(
       return &(filter.Get(i));
     }
   }
-  return NULL;
+  return nullptr;
 }
 
 void MockConnectionManager::SetServerReachable() {