ActiveTab Permissions Difference

## Detailed description

There are some differences it appears in how ActiveTab permissions are executed between Chrome and Firefox. Chrome for instance will pass permissions into same-origin iframes. Firefox on the other doesn't appear to allow you to access iframes even if it's on the same origin using the activeTab permissions. This helps us validate the page for storybook iframes where storybook itself doesn't have carbon components, but the iframe embedded does. I open up [this issue](https://bugzilla.mozilla.org/show_bug.cgi?id=1653408) against firefox to look into it. I got a temporary fix in for the time being the following two lines of permissions to the `manifest.json` file.

https://github.com/carbon-design-system/devtools/blob/master/extension/manifest.json#L9-L10