Move NamedThirdPartyRegistry initialization of blocking execution path

[AndriusA]

Description

brave/brave-core#4229 implements bandwidth savings prediction using named third parties as one of the signals for the predictive model. Third parties are looked up by domain name in NamedThirdPartyRegistry, but NamedThirdPartyRegistry is a singleton that loads mappings from JSON on first use, potentially locking up the main thread.

Steps to Reproduce

N/A

Actual result:

Histogram Brave.Savings.NamedThirdPartyRegistry.LoadTimeMS records tame taken for initialization - simple experiment on a powerful desktop shows 4ms spent.

Expected result:

Loading the bandwidth prediction component should not block request processing

Reproduces how often:

Every browser launch

Brave version (brave://version info)

Current devevlopment, not yet landed

Version/Channel Information:

• Can you reproduce this issue with the current release? N/A
• Can you reproduce this issue with the beta channel? N/A
• Can you reproduce this issue with the dev channel? N/A
• Can you reproduce this issue with the nightly channel? N/A

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? N/A
• Does the issue resolve itself when disabling Brave Rewards? N/A
• Is the issue reproducible on the latest version of Chrome? N/A

Miscellaneous Information:

[bsclifton]

Setting missing milestone to 1.10