use instance var for TLD in text zoom coordinator

[brindy]

Task/Issue URL: https://app.asana.com/0/392891325557410/1209139290185318/f
Tech Design URL:
CC:

Description:
Speculative crash fix. Crashes are coming from init of TLD but there's no need for it to do that so moving to an instance variable.

Steps to test this PR:

1. Smoke test setting the text zoom
2. Specifically go to a website, set the zoom level. Open a new tab and visit the same site.
3. Fire button should clear zoom levels. Visit the same site and check that it's not applied again.

[miasma13]

LGTM 👍 , tested it and the text zoom works correctly.

One comment for potentially unrelated change.