New HttpResponseInfo field: original_response_time

[Adam Rice]

https://chromium-review.googlesource.com/c/chromium/src/+/5937821 adds a new field to the cache metadata, original_response_time. The purpose is to correctly identify when code cache entries can no longer be used (see the issue https://crbug.com/373703283).

Since adding a new field to the metadata is irreversible, I'd like to get a few more eyes on it. PTAL if you have time.

Thanks,

Adam