Disparity between time bases in HighResolutionTime and ResourceTiming

Resource Timing says:

"...the user agent must record the system clock at the beginning of the *
root* document navigation..."

High Resolution Time says:

"...the number of milliseconds from the navigationStart attribute of the
PerformanceTiming interface, the start of navigation of *the* document..."

So in iframes, Resource Timing will be offset, but now() won't be. That
seems bad. We should be consistent.

We went with the root document in Resource Timing intentionally. We want to
present a uniform timeline that can just be displayed, including iframe
content.

So I think we should change High Res Time to match Resource Timing's
definition.

We should define "root document" too. I think we can work the cross-origin
concerns into that: walk the parent document chain as long as there's a
parent and it has the same origin as the current document. Wherever you
stop is the root document.

James