Tweak on PinnedTopSite icon to avoid overflow in new tab page

[cezaraugusto]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Ran git rebase -i to squash commits (if needed).

Fix #6281

Auditors: @luixxiul

Test Plan:

• Go to about:newtab
• Pin a site
• Pinned icon should not be overflown by favicon (should be aesthetically correct)

[luixxiul]

I cannot reproduce the issue nor confirm the fix. Will you please tell me how to?

Also is it expected?

[cezaraugusto]

on master, if you pin a defaultSite (hover over tile, hit "pin" icon) like Brave AppStore link, or any other tile that has a big favicon, pinned icon gets 'cuted' (or doesn't even appear at all).

about your mention, yes it's expected. Maybe we want to polish pinned background-color for when tile has a darker background (such as Facebook), but not related to the work I did here.