Expose map renderer type in the API

This pull request relates to issue #2004
The `goog.functions.constant` function serves two purposes that I can see:

 1. Calls to it can be minified more than an anonymous function.  Smaller transfer burden.
 2. If called in places where an anonymous function would otherwise be written in a larger closure, it can be less of a memory burden (as long as someone retains a reference to the anonymous function, everything in the closure needs to be retained as well).

In the cases I sited above, using `goog.functions.constant` is largely about the first category (minification).  Though it is tempting to think otherwise when reading parts of the source, I don't think the Closure Library authors are trying to fake anything about the language here.