Bugzilla

I have just gave a quick try to convert readerview@mozac.org from a persistent background script to an event page, as mentioned in comment 0 the only detail that needed to be adjusted is the Map kept in the background page to be using storage.session API instead.

I have then used the STRs we have put together for QA to verify the behaviors on the extension process crashes to verify that:
- the readerview event page is being suspended on idle as expected
- webpages recognized as articles successfully switch to reader mode before a crash
- and then after a crash, confirm that:
  - an existing tab in readerview mode is shown the usual "tab crashed UI" available on Fenix, and clicking "Restore tab" does restore the tab in readerview mode as it was before the process crashed
  - clicked the readerview mode on a tab that was not in readerview mode, does spawn the event page and switch the tab to the readermode view as expected

Follows the diff with the current set of changes I've applied locally:

```diff

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(was same as comment 1, duplicated by mistake).