Brave News: use a larger IntersectionObserver viewport for pagination

[petemill]

To lessen risk that the trigger element would be "skipped" over the viewport when scrolling down fast. Previously, this easily exposed the situation where further content would not be loaded since the hidden pagination trigger element had jumped from below the viewport to above the viewport, avoiding being in the viewport and triggering intersection.

This occurred for both the first "page" of content or any subsequent "pages".

Trigger position is the same (900px from the bottom of the viewport), as previously the trigger element used 900px absolute bottom position.

Resolves brave/brave-browser#13761

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:

• Open NTP
• Opt in to Brave News
• Open another NTP
• Scroll down immediately and very fast
• Content is loaded (i.e. more than 1 card is shown)
• Use the mouse to move the scrollbar immediately to the bottom.
• Verify that another page of content is loaded.
• Repeat and verify some more times. On previous versions, this easily exposed the situation where further content would not be loaded since the hidden pagination trigger element had jumped from below the viewport to above the viewport, avoiding being in the viewport and triggering intersection.

[nullhook]

lgtm

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[fallaciousreasoning]

LGTM!

[kjozwiak]

Reproduced the issue on Win 11 x64 using the following build(s):

Brave | 1.41.96 Chromium: 103.0.5060.114 (Official Build) (64-bit)
-- | --
Revision | a1c2360c5b02a6d4d6ab33796ad8a268a6128226-refs/branch-heads/5060@{#1124}
OS | Windows 11 Version 21H2 (Build 22000.795)

Example of the issue occurring using the STR/Cases outlined via both brave/brave-browser#13761 (comment) & #14170 (comment).

New.Tab.-.Brave.2022-07-18.17-27-31.mp4

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.43.29 Chromium: 103.0.5060.114 (Official Build) nightly (64-bit)
-- | --
Revision | a1c2360c5b02a6d4d6ab33796ad8a268a6128226-refs/branch-heads/5060@{#1124}
OS | Windows 11 Version 21H2 (Build 22000.795)

Couldn't reproduce the original issue using the STR/Cases outlined via brave/brave-browser#13761 (comment) & #14170 (comment).

Example of the issue not occurring and more news articles loading once hitting the bottom of the feed:

New.Tab.-.Brave.2022-07-18.17-37-21.mp4