Determine navigationStart as fallback timeOrigin

[paulirish]

Currently 0 time is selected as the first timestamp found in the trace. However this will skew results the more that there's a gap between tracing started and navigation started.

Ideally we should find the navigationStart in the trace if a timeOrigin is not provided in the options.
Doing that is fairly straightforward:

https://github.com/GoogleChrome/lighthouse/blob/master/lighthouse-core/gather/computed/trace-of-tab.js#L53-L67

[brendankenny]

@paulirish should this be closed now?

[patrickhulce]

This has actually become far more urgent now, Chrome now injects meta events into the trace that have a ts value of 0 which will cause issues like GoogleChrome/lighthouse#2114 far more frequently. At a minimum, it should probably be changed to the first non-zero timestamp of the trace (or first trace event not in the __metadata category).

The meta events are already screened for, in my case it was an issue with a page's user timing event that had been injected (like in GoogleChrome/lighthouse#2114).