We should stop tracking source => target hosts by the Referer header, and instead use frame attribution.

This can be done using the new Navigation API in //content, and we will replace the TimedCache with something like PredictorNavigationMap, which tracks all subresources requested within 10 seconds of navigation start, keyed off of frame.

We also may be able to use the ResourceThrottles themselves instead of the //content API.

Now that all requests are tracked with resource throttles, frame attribution should be easy.