Brave News: do not request ad content multiple times for the same ad unit

[petemill]

Resolves brave/brave-browser#17471

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
• 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:

• Open NTP
• Open DevTools and ensure "debug" messages are being shown. Feel free to filter to "Brave News"
  
• When scrolling down to Brave Today, ensure that only a single "Brave News: asking for an ad" is logged.

[nullhook]

LGTM

[bsclifton]

A bit late - but I did pull the code, build it, and ran through the test plan. Works great! 😄 👍

[petemill]

Thanks @bsclifton !

[stephendonner]

Verified PASSED using the test plan here and build

Brave	1.30.27 Chromium: 92.0.4515.159 (Official Build) nightly (x86_64)
Revision	0185b8a19c88c5dfd3e6c0da6686d799e9bc3b52-refs/branch-heads/4515@{#2052}
OS	macOS Version 11.5.2 (Build 20G95)

Steps:

1. new profile
2. launched Brave
3. opened the "hamburger" menu, and chose More Tools -> Developer Tools
4. from the All levels dropdown, added Verbose
5. opened a new-tab page
6. scrolled down until I saw an ad
7. examined console
8. kept scrolling
9. examined console

Confirmed I only had a single request for the initial ad: asking for an ad 0; in the logs in that screenshot, is shown asking for an ad 1, after I scrolled down a bit (thus queuing up the next ad).

example	example

---

Verification PASSED on Win 10 x64 using the following build:

Brave | 1.30.38 Chromium: 93.0.4577.51 (Official Build) nightly (64-bit)
--- | ---
Revision | 762d21050e2da59930c784c09b134d0b0b148188-refs/branch-heads/4577@{#915}
OS | Windows 10 OS Version 2009 (Build 19042.1165)