DevTools: Use StringMap instead of {} in SettingsList.

This will fix odd errors if "__proto__" is used as a regex pattern or path prefix.

R=vsevik

Review URL: https://codereview.chromium.org/421923004

git-svn-id: svn://svn.chromium.org/blink/trunk@179221 bbb929c8-8fbe-4397-9dbb-9b2b20218538