Update default for magic titlebar on Windows/Linux; add favicon for about pages

[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).

• Update default value for magic titlebar

  • false for Linux (ex: always show URL bar)
  • true for Windows (ex: use magic titlebar)

• add favicon for about pages for which it was missing

  • about:autofill
  • about:passwords

Auditors: @BradRichter, @darkdh

Fixes #5416 (URL bar duplicates the title bar)

part 1: linux

1. Install Brave (fresh install or move session folder) on Linux
2. Launch Brave and visit brave.com
3. URL bar should always be showing
4. Go to about:preferences and ensure "Always show URL bar" is enabled

part 2: windows

1. Install Brave (fresh install or move session folder) on Windows
2. Launch Brave and visit brave.com
3. URL bar should be hidden unless mouse is in there
4. Go to about:preferences and ensure "Always show URL bar" is disabled

NOTE: this PR does NOT fix #2301; keep that in mind when testing. Windows may have the correct setting, but still show the URL bar ☹️

Fixes #5398 (about:autofill missing Brave favicon)

1. Launch Brave and visit about:about
2. Click about:passwords and notice the favicon now shows up
3. Click about:autofill and notice the favicon now shows up

[darkdh]

++

[bsclifton]

Just updated the Windows setting 😄 so after this is merged, new Windows users will get to experience the magic titlebar! (which is great, since the slim titlebar is finished and working well)

[bbondy]

I didn't even realize we didn't have it on Windows, hah.