lib: remove variables root and GLOBAL

The `root` and `GLOBAL` never be documented.

Author: JacksonTian

src/node.js

@@ -159,8 +159,17 @@
   startup.globalVariables = function() {
     global.process = process;
     global.global = global;
-    global.GLOBAL = global;
-    global.root = global;
+
+    // Deprecate GLOBAL and root
+    const holders = {GLOBAL: global, root: global};
+    ['GLOBAL', 'root'].forEach(function (name) {
+      const access = NativeModule.require('util').deprecate(function(val) {
+        if (val) holders[name] = val;
+        return holders[name];
+      }, "'" + name + "' is deprecated, use 'global'");
+      Object.defineProperty(global, name, {get: access, set: access});
+    });
+
     global.Buffer = NativeModule.require('buffer').Buffer;
     process.domain = null;
     process._exiting = false;