The purpose of this commit is to implement https://w3c.github.io/resource-timing/#dfn-timing-allow-check

The purpose of this commit is to ensure that the Response object has
access to Timing updates as previously the Response object simply
stored a ResourceFetchTiming struct so updates on ResourceFetchTiming
that were not explicitly done on the Response would not be passed down.
The references to ServoArc are added because Response uses
servo_arc::Arc rather than std::sync::Arc as is used elsewhere. So,
we've switched those other places to servo_arc::Arc instead of switching
Response to std::sync::Arc.

The purpose of this commit is to expose stub attributes on
PerformanceResourceTiming in order to enable tests to pass as well as
set the type of performance timing entry to navigation when the
initiator is navigation. The purpose of this is to ensure that
document performance entries are correctly classified as navigation
entries rather than resource entries.