[Security] Enable blocklist_extension_fetcher

[jumde]

Source here: https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/extensions/blocklist_state_fetcher.cc

Community reports:

• https://community.brave.com/t/brave-is-not-disabling-or-marking-extensions-that-have-been-removed-from-the-chrome-web-store-as-malware/112122
• https://community.brave.com/t/privacy-and-security-bug-brave-not-disabling-extensions-removed-from-the-chrome-web-store-as-malware-e-g-nano-defender/173533
• https://www.reddit.com/r/brave_browser/comments/jgifxa/privacy_and_security_bug_brave_not_disabling/

[fmarier]

Looks like #4674 will be needed first since the Google endpoint is https://safebrowsing.google.com/safebrowsing/clientreport/crx-list-info which we don't currently have a proxy for.

[fmarier]

There's no easy way to test the extension blocking functionality, but it's covered somewhat by a unit test.

What I would suggest instead as a test plan is to ensure that we didn't break download protection:

1. Open https://testsafebrowsing.appspot.com/
2. Check that all links in the "Desktop Download Warnings" lead to blocked downloads.

[LaurenWags]

Verified using

Brave	1.21.64 Chromium: 88.0.4324.182 (Official Build) beta (x86_64)
Revision	73ee5087001dcef33047c4ed650471b225dd8caf-refs/branch-heads/4324@{#2202}
OS	macOS Version 10.15.7 (Build 19H512)

Verified test plan from #12297 (comment). Confirmed all links under "Desktop Download Warnings" were blocked.

Example	Example

---

Verification passed on

Brave | 1.21.63 Chromium: 88.0.4324.182 (Official Build) dev (64-bit)
-- | --
Revision | 73ee5087001dcef33047c4ed650471b225dd8caf-refs/branch-heads/4324@{#2202}
OS | Windows 10 OS Version 2004 (Build 19041.804)

Example	Example

---

Verification passed on

Brave	1.21.64 Chromium: 88.0.4324.182 (Official Build) beta (64-bit)
Revision	73ee5087001dcef33047c4ed650471b225dd8caf-refs/branch-heads/4324@{#2202}
OS	Ubuntu 18.04 LTS

Verified test plan from #12297 (comment)