chore(rum-core): use startTime for LCP marks

[vigneshshanmugam]

• shorter to use startTime as it fallbacks automatically to renderTime if its available or loadTime.
• More details - https://wicg.github.io/element-timing/#sec-performance-element-timing. LCP is part of element timing.

[codecov-commenter]

Codecov Report

Merging #815 into master will decrease coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
- Coverage   93.37%   93.05%   -0.33%     
==========================================
  Files          50       50              
  Lines        2445     2303     -142     
  Branches      516      460      -56     
==========================================
- Hits         2283     2143     -140     
+ Misses        159      157       -2     
  Partials        3        3              

Impacted Files	Coverage Δ
...ges/rum-core/src/performance-monitoring/metrics.js	89.53% <100.00%> (-2.84%)	⬇️
.../src/performance-monitoring/transaction-service.js	90.69% <0.00%> (-2.99%)	⬇️

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #815 opened]

• Start Time: 2020-06-15T13:29:28.924+0000

• Duration: 77 min 33 sec

Test stats 🧪

Test	Results
Failed	0
Passed	938
Skipped	10
Total	948

Steps errors

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• Name: Bundlesize

  • Description: #!/bin/bash set -o pipefail npm run bundlesize|tee bundlesize.txt

  • Duration: 1 min 39 sec

  • Start Time: 2020-06-15T13:36:10.647+0000

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