Remove border radius from Windows 7; it has issues on higher DPI setups.
Remove border radius from Windows 7; it has issues on higher DPI setups.

Auditors: @bbondy

Test Plan:
1. Launch Brave on Windows 7
2. Ensure the top left and top right corners are not rounded with a black outline.
   They may be rounded with NO outline (and that's OK)
3. Crank DPI up to 150%
4. Ensure top left/right borders are square and look correct

luixxiul/browser-laptop,luixxiul/browser-laptop,willy-b/browser-laptop,Sh1d0w/browser-laptop,jonathansampson/browser-laptop,dcposch/browser-laptop,darkdh/browser-laptop,willy-b/browser-laptop,dcposch/browser-laptop,jonathansampson/browser-laptop,diasdavid/browser-laptop,luixxiul/browser-laptop,dcposch/browser-laptop,pmkary/braver,timborden/browser-laptop,Sh1d0w/browser-laptop,diasdavid/browser-laptop,willy-b/browser-laptop,jonathansampson/browser-laptop,jonathansampson/browser-laptop,timborden/browser-laptop,pmkary/braver,luixxiul/browser-laptop,pmkary/braver,timborden/browser-laptop,diasdavid/browser-laptop,darkdh/browser-laptop,willy-b/browser-laptop,Sh1d0w/browser-laptop,timborden/browser-laptop,Sh1d0w/browser-laptop,pmkary/braver,diasdavid/browser-laptop,dcposch/browser-laptop,darkdh/browser-laptop,darkdh/browser-laptop