Show the caption buttons when in full screen mode (Windows only)

[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). N/A
• Ran git rebase -i to squash commits (if needed).

Show the caption buttons when in full screen mode (Windows only)

Fixes #4376

Auditor: @bbondy

Test plan:

1. Launch Brave
2. Go full screen by pushing F11
3. Notice the caption buttons are showing
4. Use the caption buttons to exit

[bbondy]

++