🐛 [RUMF-1344] try to improve document scrolling element handling

[bcaudan]

Motivation

Following #1680, some telemetry events show that some documents seems to lack scrolling element:

• specific debug log: 'unable to find document scrolling element'
• error on setting null as a weak map key

I have not been able to reproduce the behavior while going on available customer pages.

Changes

• access document.scrollingElement only on document scroll
• try to fallback on document.documentElement
• improve logging

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #1688 (0c65a8d) into main (1a8d1cf) will decrease coverage by 0.30%.
The diff coverage is 19.04%.

@@            Coverage Diff             @@
##             main    #1688      +/-   ##
==========================================
- Coverage   90.75%   90.45%   -0.31%     
==========================================
  Files         128      128              
  Lines        4739     4755      +16     
  Branches     1059     1065       +6     
==========================================
  Hits         4301     4301              
- Misses        438      454      +16     

Impacted Files	Coverage Δ
...s/rum/src/domain/record/elementsScrollPositions.ts	41.37% <19.04%> (-50.93%)	⬇️

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[liywjl]

Unit tests might be worth adding