Content script is not executed on nested iframes

[dedoussis]

How to reproduce:

1. Navigate to https://eoft.es/
2. Click on Girona
3. Attempt to buy a ticket
4. In the checkout form, attempt to autofill the email via the context menu. An alias is generated and reserved, but is not populated in the html input element.

RCA: The checkout form is a nested iframe. The content script of an extension is not injected on nested frames unless the all_frames: true attribute is set in manifest.json: https://stackoverflow.com/questions/3937355/accessing-iframes-from-a-chrome-content-script-extension?rq=4