Since the site-specific channels feature was turned on by default, notifications are only posted to the generic 'Sites' channel if the site-specific channel could not be found*, as a fallback.

However, this should not be necessary, since notification permission is checked whenever a notification is to be displayed, and notification permission for a site on Android O+ is only reported as 'granted' if a site-specific notification channel exists and is enabled.

The remaining thing to decide is whether to put in any mitigations in case for some unexpected reason a channel *doesn't* actually exist for the site.

We should probably at least add an 'assert' that the channel is not null here before using it, for better traces in debug builds:

https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/notifications/channels/SiteChannelsManager.java?q=SiteChannelsMan&sq=package:chromium&l=183

Other ideas include:

1. Send ourselves UMA if this codepath is being used
  - downside: it will take a long time for this UMA to reach stable and tell us anything useful

2. Fall back to another channel, e.g. the 'Browser' channel.
  - downside: Probably not good to label something coming from a site as coming from the browser

*Or if the kSiteNotificationChannels feature flag is disabled, but this flag will shortly be removed in https://crrev.com/c/868654