New tab shows letterboxing size indicator

In !1469 (comment 3184766) this was addressed in a way that was no longer noticeable on my regular system. But when testing in my virtual machine, which is much slower, it is still noticeable.

Steps to reproduce:

1. Open tor browser.
2. Resize window a little.
3. Click "Configure connection", which open about:preferences#connection. Or click the tab "+" button.

Alternative steps:

1. Open tor browser and bootstrap.
2. Enable the setting "When you open a link, image or media in a new tab, switch to it immediately".
3. Visit example.org.
4. Open the link in a new tab.

The letterboxing size indicator is briefly shown before being hidden when the window content loads. This is because the blank window initially does have letterboxing applied.

In the first steps, it should not be shown because the new page ultimately does not use letterboxing. In the second step, it should not be shown because we are switching between two sites that both use the same letterboxing, so there is no need to remind the user of the same size.

I'm finding it hard to judge how much this would be noticeable in general because I don't know what this would look like with a typical CPU.

But ideally the indicator would never show at all until we know whether the new tab starts loading enough content for us to know whether it is a letterboxing tab or not. I.e. we can apply the initial letterboxing, but we should only show the indicator when it is definite and a resize.

/cc @ma1