It's possible for GCM registration/InstanceID tokens to be invalidated server-side without Chrome's knowledge, for example if the device is left in a drawer powered off for N months.

In such cases, a website that follows best practice by frequently calling PushManager.subscribe() to ensure its subscription is still valid would break, because Chrome would keep returning the invalid cached token.

The Android GCM client works around this class of issues by only caching tokens for a week, after which calls to subscribe will hit the server again (see https://goto.google.com/hxejm).

We should do the same. These tokens are cached both in Chrome GCM code and Push code, so we'll need to fix multiple layers of caching.