Trace minifier script should keep the first screenshots after FP/FCP/LCP

**Summary**
In https://github.com/GoogleChrome/lighthouse/pull/11874#discussion_r558590089 I was doublechecking that the FCP value was the correct one but the throttled screenshots from trace minifier script discarded the important first frame. The minifier should always keep the first screenshot after each FP/FCP/LCP event.

Locate the trace minifier script and inspect how it selects screenshots around FP, FCP, and LCP events. Verify the behavior against a trace containing those events; done means the first screenshot after each event is retained rather than discarded.

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