AdsServiceFactory::GetForProfile returned nullptr when IsSupported
was false, which includes a managed pref that can change during a
session. The guard is removed from the factory so AdsService is
always created for regular profiles. BAT ads startup is now gated
in CanStartBatAdsService instead. AdsServiceImpl and
AdsServiceDelegate also handle null service dependencies gracefully.
A follow-up will add an ads-specific group policy to allow
independent control of ads without relying on the Rewards policy.
Terry Mancey
0dbf7fd2da