[Brave News]: Fix crash with non-utf-8 RSS feeds.

[fallaciousreasoning]

Resolves brave/brave-browser#26321

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

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, 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-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[sangwoo108]

LGTM 👍

[stephendonner]

Verified PASSED using

Brave	1.47.36 Chromium: 107.0.5304.91 (Official Build) nightly (x86_64)
Revision	3d5948960d62418160796d5831a4d2d7d6c90fa8-refs/branch-heads/5304@{#1097}
OS	macOS Version 11.7.1 (Build 20G918)

Steps:

1. install 1.47.36
2. launch Brave
3. set brave://flags/#brave-news-v2 to Enabled
4. set brave://flags/#brave-news-subscribe-button to Enabled
5. click Relaunch
6. load https://www.pravda.com.ua/eng/news/

Confirmed no crashes when loading https://www.pravda.com.ua/eng/news/ with the above flags enabled

[petemill]

This std::move of body_content is problematic since it means body_content is empty when accessed later in the callback if the content was HTML and not RSS. I'm pretty sure it's causing brave/brave-browser#26550