first trace event can be far off from navStart

[patrickhulce]

discovered in GoogleChrome/lighthouse#2095

Traces can sometimes contain events that are far before navStart, even in more regular cases I've seen screenshots from the previous page linger in a trace of the new page that throws off the computation.

Two proposals with varying degrees of urgency:

1. filter out screenshots before the user-provided timeOrigin opt (PR: #2114)
2. consider defaulting startTs to the last occurrence of navigationStart for the root frame instead of the first event of the trace (Determine navigationStart as fallback timeOrigin #38)