Hmm? NaCl is a restricted runtime environment once supported by Google Chrome (i.e. web browser) to run binary programs in a way similar to native applications - but inside Chrome. Someone (Matto?) even used this strange thing eventually. It somewhat sandboxes and virtualises OS though, providing virtualized facilities rather than direct OS access. Actually these are compiled programs, though later PNaCl gone for "abstract" LLVM bytecode rather than native platform code that is converted to native code prior to running it much like clang does with intermediate code transform during compilation.
I've got impression M+ only been using that thing to distribute via google chrome addons store - as long as google supported this option. And I've got impression M+ for android is more or less native build, that got nothing to do with that "NaCl" thing at all. I guess it needs something like NDK to build as native program, many games do it like this. I can't find mention of NaCl in anything related to Android. Maybe someone can show me that part in code, once and for all? I'm not android expert but still curious where this odd idea comes from.