[MD settings] move extra options in category settings
This CL refactors the site settings category code to move one-off
details from that code. This was done by moving much of the code to a
new category default setting module. The site_settings_category has been
replaced with content_setting_exceptions.
related to bug 662596
BUG=None
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation
Committed:
https://crrev.com/891846bf42988a708ce18472546e1361387507a6
Cr-Commit-Position: refs/heads/master@{#439243}