Bug 1575889 - Fixing a bit about how we flip the pref from AS to DS

[ScottDowne]

To test:

1. Set browser.newtabpage.activity-stream.discoverystream.enabled to false
2. Open a new tab, keep it open.
3. Flip browser.newtabpage.activity-stream.discoverystream.enabled from false to true
4. Look at the tab you opened, expected, it shouldn't have changed.
5. Open a new tab
6. expected, it should be different

[Mardak]

Will strange things happen if the config doesn't exist in the feed and someone tries to interact with the "stale" page? E.g., scrolls down to a spoc which then tries to trigger an impression ?

[gvn]

Taskcluster failed but I think it's not because of your code and related to: https://bugzilla.mozilla.org/show_bug.cgi?id=1575774

[gvn]

With a fresh profile I'm not seeing anything different between the tabs at step 6.

[ScottDowne]

@Mardak Should be good. I tested a bunch of stuff to be sure.

@gvn I just tested it with a new profile and it worked fine. I think maybe we can test this over zoom in a bit?

[ScottDowne]

Found some bugs in pref change acting like a startup in a few cases, causing visual update to the open tabs.

Todo for me.

1. File a new bug.
2. Force push to fix taskcluster.

[gvn]

Current test steps are working 👍