Fix alignment of home button between tab with content and about:newtab

[bsclifton]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Fixes #4854

Auditors: @jkup @bbondy

@jkup this is a proposed fix which has the same symptom as the favorites button you had adjusted with #4726. This PR should fix it too- basically, it's putting a placeholder button in place (which has opacity 0) which causes the flexbox to adjust it's bounds properly. I'm curious if there's a better solution, but I would need to dig in more (I don't have enough CSS knowledge at the moment to know an alternative)

Test plan:

1. Launch Brave and open https://brave.com and also a new tab
2. Open preferences, enable home button
3. switch between brave.com and the new tab screen. home icon should not move

[jkup]

This is interesting, I just asked a question about that line over here #4726 (comment)

Do we even need to render an empty browserButton?

[bsclifton]

Closing in favor of solution provided w/ #4726