window.confirm no longer works on about: pages

[diracdeltas]

• Describe the issue you encountered:
  window.confirm works usually on regular pages but never on about: pages, it seems. this breaks clearing passwords on about:passwords.
• Platform
  • Win7, 8, 10? OS X? Linux(distro)?
    osx
• Brave Version
  git head, 0.9.6
• any related issues
  deleting database broken on fmphost.com #1786 window.prompt is not supported currently #94

[bridiver]

fixed brave/muon@22312fc