Revert "app.makeSingleInstance is not available on OS X"

This reverts commit bcb78ebc00.

atom/browser/api/atom_api_app.cc

@@ -288,10 +288,6 @@ v8::Local<v8::Value> App::DefaultSession(v8::Isolate* isolate) {
 
 bool App::MakeSingleInstance(
     const ProcessSingleton::NotificationCallback& callback) {
-#if defined(OS_MACOSX)
-  LOG(ERROR) << "MakeSingleInstance is not implemnted on OS X";
-  return false;
-#endif
   if (process_singleton_.get())
     return false;
 

docs/api/app.md

@@ -290,7 +290,7 @@ URLs that fall under "Local Intranet" sites (i.e. are in the same domain as you)
 However, this detection often fails when corporate networks are badly configured,
 so this lets you co-opt this behavior and enable it for all URLs.
 
-### `app.makeSingleInstance(callback)` _Windows_ _Linux_
+### `app.makeSingleInstance(callback)`
 
 * `callback` Function