Define goog and assign to global when generating UMD debug builds

When concatenating the Closure Library, `base.js` creates a new `goog` object if there is not already one in scope.  Later, `goog.global` is assigned the value of `this`.  Calls to `goog.provide` create "namespace" objects by assigning to `goog.global`.  To ensure that `goog` is the same as `goog.global.goog`, we need to create a new `goog` object in the scope of base.js and assign it to `this.goog`.