I think it should be "const base::Value&". The caller who iterates over the base::ObserverList can retrieve a value and let every observer read it. Observers can always base::Value::DeepCopy() it for themselves if they need to change it.

Seems legit. Assigning back to reporter to get out of triage queue.

Changing to available. I asked csharrison@ to document this problem, but he should feel no obligation to fix it :)

I'll take this one. I'll replace the existing content_settings::Observer::OnContentSettingChanged method in-place.

Due to a recent reorg, I'm unlikely to be able to take this now.

Circling back to this one.
My new proposed API is this:
virtual void OnContentSettingChanged(
    const ContentSettingsPattern& primary_pattern,
    const ContentSettingsPattern& secondary_pattern,
    ContentSettingsType content_type,
    std::string resource_identifier,
    const base::Value& old_value,
    const base::Value& new_value) = 0;

It seems that this API is called quite often if a setting is "changed" to the same exact value, so my use-case would first compare old_value to new_value and make sure the change was semantically meaningful.

Some OnContentSettingChanged calls don't correspond to individual changes. old_value and new_value wouldn't be defined in these cases. How could we represent this? base::Optional<base::Value>? 
If we are only interested in changed values of actual ContentSettings, we could also pass base::Optional<ContentSetting>?

E.g.
https://cs.chromium.org/chromium/src/chrome/browser/extensions/api/content_settings/content_settings_custom_extension_provider.cc?l=49&rcl=974d9b8cd88fdb46a4f9a783311696253f183edc
or
https://cs.chromium.org/chromium/src/components/content_settings/core/browser/content_settings_policy_provider.cc?l=461&rcl=f355f667c9ec1f618568af2e60d755f74cefe903

dullweber: Yeah I'm not 100% confident about the internal implementation of the API, but for changes which span multiple content settings types, I could imagine a separate observer method.

It looks like in the two links you posted, we are changing the default setting type, but I'm not sure exactly what that means :)

I think these calls just mean "something might have changed" e.g. because an extension was uninstalled that controlled content settings or a policy was changed.