Use a global singleton instance of referrals list everywhere

[iefremov]

Follow-up for #3176
BraveNetworkDelegateBase listens for updates of the referral headers list, but this is actually not needed. Since the list is a global for the whole app we can just get it when needed.

This way, we can remove unsafe usages of base::Unretained in BraveNetworkDelegateBase. To safely get the list on IO thread, I'd suggest to keep a copy of a the list (or a single instance if nobody wants it on other threads) and atomically update this copy during updates

Brave version (brave://version info)

Brave	0.62.0 Chromium: 72.0.3626.71 (Developer Build) (64-bit)
Revision	f52ccad2a6a3c65fc9e0c591a517ceab1198dac0-refs/branch-heads/3626@{#763}

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[iefremov]

@bbondy @emerick @bridiver FYI