[Resource Timing] Monotonic Clock

I was reading through the Resource Timing spec again looking at the
monotonic clock requirements. I had a couple of comments on it.

First, it says that "timing attributes are not skewed by adjustments to the
system clock during the navigation." We might not be navigating when
resources load, but they should still use a monotonic clock referenced from
the initial page load so that we can construct an accurate timeline.

Also, what do we do about resources that load from iframes? Are they
referenced from the iframe's navigation or from the root navigation? The
latter seems most useful from my perspective, but maybe there are other
concerns there.

James