Resource Timing API defines this:
“If there are no HTTP redirects or equivalent, this attribute must return the time immediately before the user agent starts to fetch the resource.”
What could be a corresponding field in the WPT HAR file? This event definitely happens before the dns lookup starts. But, the lowest start value in WPT for a given resource is _all_start which, I think, cannot be less than _dns_start and so I am not sure if WPT provides this metric at all.