Fixes rewards-image handler post C80 bump (1.3.x).

[mkarolin]

Fixes brave/brave-browser#8074
Uplift from #4514

Submitter Checklist:

• 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
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• 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-Protection-Mode
  • 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.

[simonhong]

LGTM

[kjozwiak]

Both Linux & macOS failed audit-network which is a known issue and not specific to this PR:

• Linux - * https://ci.brave.com/job/brave-browser-build-pr/job/maxk-fix-rewards-image-1.3.x/1/execution/node/562/log/
• macOS - https://ci.brave.com/job/brave-browser-build-pr/job/maxk-fix-rewards-image-1.3.x/1/execution/node/639/log/

macOS failed test-install which is a known issue and not specific to this PR:

• macOS - https://ci.brave.com/job/brave-browser-build-pr/job/maxk-fix-rewards-image-1.3.x/1/execution/node/698/log/

[kjozwiak]

Uplift into 1.3.x approved after deliberating with @brave/uplift-approvers. As per #4522 (comment), audit-network failed on both Linux & macOS and test-install failed on macOS. These are known failures/issues that are not related to this specific PR.

Please make sure that all the correct labels are being used and the associated issue is moved into the correct milestone.