Make Settings.in invalidation handling generic

This is in preparation for deleting a bunch of boiler-plate
code in Settings.cpp now that we have a SettingsDelegate
and a generic (enum-based) way to inform it of
Settings invalidations.

This is not strictly related to OOPI, this is just a
victory-lap patch following:
https://codereview.chromium.org/111963005

BUG=305811
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/101623008

git-svn-id: svn://svn.chromium.org/blink/trunk@163818 bbb929c8-8fbe-4397-9dbb-9b2b20218538