Moved the JS content handler, the gl and monotonic_clock modules from mojo/apps/js to services/js.

Added the following main directories:
mojo/services/public/js - The JS content handler client module.
services/js - The JS content handler implementation.
services/js/modules - The gl and clock modules.
examples/js - Mostly a TBD.

Added js_services_unittests.

This should the last big file shuffle for JS in Mojo.

BUG=430520
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/703023004