Right now, when an extension wants to add support for contextual identities, the only way to do so is to put `contextualIdentities` into the `permissions` manifest.json field. This however forces the Contextual Identities feature to be enabled with the only way to turn it off being to disable the extension. Some extensions don’t need to always have contextual identities enabled, so in this case, the `contextualIdentities` permission would instead be declared in the `optional_permissions` field, which would make it so that the Contextual Identities preference wouldn’t be forcefully enabled.