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

mojo/system/memory.h

Index: mojo/system/memory.h
diff --git a/mojo/system/memory.h b/mojo/system/memory.h
index 3602be74d2734333b6b51853850b63d9193bb058..acbbb5421b94177f373b7ca959102a90806b24c3 100644
--- a/mojo/system/memory.h
+++ b/mojo/system/memory.h
@@ -91,15 +91,15 @@ class UserPointer {
   // |MakeUserPointer()| (or |NullUserPointer()| for null pointers). (The common
   // exception is when you have, e.g., a |char*| and want to get a
   // |UserPointer<void>|.)
-  UserPointer() : pointer_(NULL) {}
+  UserPointer() : pointer_(nullptr) {}
   explicit UserPointer(Type* pointer) : pointer_(pointer) {}
   // Allow implicit conversion from the "null user pointer".
-  UserPointer(NullUserPointer) : pointer_(NULL) {}
+  UserPointer(NullUserPointer) : pointer_(nullptr) {}
   ~UserPointer() {}
 
   // Allow assignment from the "null user pointer".
   UserPointer<Type>& operator=(NullUserPointer) {
-    pointer_ = NULL;
+    pointer_ = nullptr;
     return *this;
   }