Bug 1657874 - Unify printer list implementations between Mac and GTK, and add nsPrinterListBase. r=jwatt

nsPrinterListBase is where the CC setup and promises and such will be, I
think.

Differential Revision: https://phabricator.services.mozilla.com/D86336

widget/windows/nsDeviceContextSpecWin.cpp

@@ -516,8 +516,6 @@ nsPrinterListWin::~nsPrinterListWin() {
   GlobalPrinters::GetInstance()->FreeGlobalPrinters();
 }
 
-NS_IMPL_ISUPPORTS(nsPrinterListWin, nsIPrinterList)
-
 NS_IMETHODIMP
 nsPrinterListWin::GetSystemDefaultPrinterName(nsAString& aName) {
   GlobalPrinters::GetInstance()->GetDefaultPrinterName(aName);