Explicitly cancel blocked requests

[spinda]

Resolves brave/brave-browser#10063

Fixes, for example, video playback on https://abcnews.go.com/Politics/video/tennessee-celebrates-100-years-womens-vote-72455124, where returning an empty stub response for https://abcnewsplayer-a.akamaihd.net/player/2.106.5/akamai/amp/nielsen/Nielsen.min.js causes another script to misbehave.

Submitter Checklist:

• [x ] Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
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• Verified that these changes pass automated tests (unit, browser, security tests) on
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• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[antonok-edm]

@spinda You'll probably need to update some of the browsertests - there's at least one that uses an $explicitcancel rule in components/brave_shields/browser/ad_block_service_browsertest.cc

[iefremov]

also any new tests are welcome, especially browsertests :) The change looks good

[spinda]

I ended up removing the explicitcancel test and updating the other tests so that they all expect resources to be blocked explicitly where they previously expected blank responses.

[bsclifton]

cc: @iefremov for re-review 😄

[bsclifton]

@spinda could you give this a rebase? Sorry that it's taken a while to work through this PR ☹️

[spinda]

Oops, accidentally deleted the branch! Rebasing this.

[pes10k]

@spinda it looks like this needs to be rebased again?

[spinda]

@pes10k Rebased again!

[pes10k]

Great! Thanks @spinda ! Once its been reviewed by @iefremov i think we're good to merge!

[pes10k]

Merged! Thanks again @spinda !