Bug 1602463 added a new background agent to Windows. We don't need it but the installer needs to be changed because it is now missing some definitions.
Also needed to add the variable BrandProductName from
Bug 1548612 Part 0 "Pre-land new strings for the revised stub installer design".
Pushed by firstname.lastname@example.org:
Port Bug 1602463 Part 4 "Register/unregister the default browser agent scheduled task during install/uninstall". r=IanN