This is some code cleanup in anticipation of moving chromeos
proxy configuration related code out of src/chrome so that
it can be accessed by networkingPrivate and ash.

This CL also namespaces some pseudo-prefs in proxy_cros_settings_parser.cc which were in the global 'chromeos' namespace before, which was super confusing because they aren't actually stored in the pref store, just set to look like prefs for the (soon to be deprecated) options UI code.

BUG=658015

Review-Url: https://codereview.chromium.org/2445153002
Cr-Commit-Position: refs/heads/master@{#427824}