Steps for calculating LCP:

1. When the pref for largest_contentful_paint_enabled is enabled, LargestContentfulPaintCandidateCollector is created once per LayoutThread using viewport_size.
2. While building the display_list, it send the pre-calculated clip_rect and bounds to LCPCollector along with transform and LCPCandidateType for visual_rect calculation for LCPCandidates.
3. After visual_rect calculation, update the lcp_candidate based on area of visual_rect and send it to compositor.
4. Then among lcp_candidates, we find out the largest_contentful_paint and send it to paint_metric
5. In addition to above LCP also uses PaintMetricState to save console from spamming with duplicate entries.

This PR includes following commits better segregation of code

1. Add LargestContentfulPaintCandidate struct
2. Add LargestContentfulPaintCandidateCollector
3. Collect the LargestContentfulPaintCandidateswhen feature is enabled.
4. Send the LargestContentfulPaintCandidates to compositor
5. Add LargestContentfulPaintCalculator
6. Calculates the LargestContentfulPaint
7. Send LCP to PaintMetrics
8. Disable LCP feature on user interaction

Elements considered for LCP in this PR:

• Image element
• An element with background image

Reference: #38041
Testing: Tested using perf-analysis-tools See attached screenshot.
Fixes: None

Perfetto

Console Output of Metric