[RUM-8744] CLS tracking: provide cls devicePixelRatio

[anediaz]

Motivation

https://datadoghq.atlassian.net/browse/RUM-8744

Implementation of small adjustments in Browser SDK for CLS attribution:

• Include devicePixelRatio attribute:
  • Recent discoveries showed that Layout Instability API does not report viewPort coordinates for prevRect and currentRect.
  • We'd need to include this data in _dd.cls.device_pixel_ratio. See this document for more context

Changes

• Include new attribute devicePixelRatio in the CLS attribution

Testing

• Local
• Staging
• Unit
• End to end

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I have gone over the contributing documentation.

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