We attach it to WorkerPrivate and DOMNavigationTiming so it will be re-used
when it should.
WorkerPrivate is used in the Performance APIs, Performance Storage Worker,
and Event.
DOMNavigationTiming is used only in the Performance APIs, but the crucial
part is that when the individual DOMNavigationTiming object is re-used,
so will the context seed. This in particular came up with the
nav2_test_document_open.html Web Platform Test which illustrated the fact
that even if you .open() a new document, the performance navigation data
is not supposed to change.
MozReview-Commit-ID: GIv6biEo2jY
