Enable 1PES domain block functionality on 20% nightly.

[goodov]

No description provided.

[kjozwiak]

Verification PASSED on Win 11 x64 using the following build:

Brave | 1.38.1 Chromium: 99.0.4844.45 (Official Build) nightly (64-bit)
-- | --
Revision | edbc0b8343c7b10fddb0e1b4efb280b0f6e38cab-refs/branch-heads/4844@{#788}
OS | Windows 11 Version 21H2 (Build 22000.527)

I went through one case just to make sure that the basics are working when a user is opted into this study via FirstPartyEphemeralDomainBlockStudy:Enabled. QA went through both brave/brave-browser#19099 (comment) & brave/brave-core#12089 (comment) as well.

Example (Default)	Example (Enabled)

Test Case

• visited https://adnium.com which was retrieved from https://pgl.yoyo.org/adservers/serverlist.php?hostformat=adblockplus&mimetype=plaintext
• once the website loaded, ensured that it was added into Sites that clear cookies when you close them under brave://settings/cookies
• ensured that there was nothing in brave://settings/siteData (no cookies/data belonging to the visited website)
• ensured that localStorage was empty for https://adnium.com via DevTools
• ensured that once the website was closed, https://adnium.com is removed from Sites that clear cookies when you close them under brave://settings/cookies

Example	Example	Example	Example