Mojo: NULL -> nullptr in mojo/system and mojo/embedder.

mojo/system/handle_table.cc

Index: mojo/system/handle_table.cc
diff --git a/mojo/system/handle_table.cc b/mojo/system/handle_table.cc
index e05e365f242614bf96fc5b38d2d64461a59dd5e7..5c7c74c3abea6627c0e9989c23702434cad89a15 100644
--- a/mojo/system/handle_table.cc
+++ b/mojo/system/handle_table.cc
@@ -36,7 +36,7 @@ Dispatcher* HandleTable::GetDispatcher(MojoHandle handle) {
 
   HandleToEntryMap::iterator it = handle_to_entry_map_.find(handle);
   if (it == handle_to_entry_map_.end())
-    return NULL;
+    return nullptr;
   return it->second.dispatcher.get();
 }