Chrome configuration: Harmony features in Chrome

This PR adds the flag to enable ES6 / Harmony features in Chrome. 
The flag is equivalent to enabling *Experimental JavaScript* via **chrome://flags/**. It enables "proxies, weak maps, sets, maps, typeof semantics, and block scoping" (see https://plus.google.com/+PaulIrish/posts/T615Md5JPQG).

I'd like to add this flag as I'm currently using ES6 proxies in a [LivelyKernel branch](http://github.com/LivelyKernel/LivelyKernel/tree/object-versioning) and would like to add my tests to CI runs of that branch.

Can we add this?

@rksm @bastiansteinert