Unified extension menu button is present in popup window's toolbar, occupying pr...

1. Load attached testcase.
2. Click the button (to open a popup viewing https://example.org )
3. Look at the toolbar on the popup window.

ACTUAL RESULTS:
The popup window's toolbar has the unified extension-menu button.

EXPECTED RESULTS:
No unified extension-menu button should be shown in the popup window's toolbar.

A few additional supporting points for why we shouldn't show this button:

• I could imagine there being a case for this button to be present if it was a way for users to e.g. activate content-modifying extensions (Stylus, uBlock Origin, etc.). However, those extensions are not listed in this menu at all in popup windows (despite showing up in the menu for a regular window). [edit: see comment 3 for more on this]
• Space is typically quite-constrained in popup-windows, to the point that we don't even show a "back" button or a "reload" button. It's unlikely we want to reserve space for this extension menu if we aren't reserving space for a back button.
• I ran into a real-world example where we've simply run out of space in the toolbar of a popup window, and removing this button would address that case. (See bug 1827031; I'm marking it as a dependency since I'm guessing that we might fix that one by fixing this bug here.)