Mojo JS Bindings: Enable ES6 classes, add an Application base class.

The JS content handler now uses the new ES6 "Harmony" support for JS classes.

Added an Application base class that creates Shell and ServiceProvider (for acceptConnection) objects. Converted tests and examples to Application, classes.

BUG=
R=aa@chromium.org

Review URL: https://codereview.chromium.org/774933010