Design doc for No-Vary-Search support in the HTTP disk cache

[Adam Rice]

The annoyingly-named No-Vary-Search response header enables a server to indicate which query params are to be ignored for the purposes of caching.

Example:

No-Vary-Search: params=("usp")

would indicate that the `usp` query parameter makes no difference to the appearance and functionality of the page or resource and so URLs can be treated as equivalent for caching purposes regardless of its value or whether it is present or not.

The user benefit is improved cache utilization resulting in better responsiveness.

My project is to implement support for this in the HTTP disk cache in //net.

Please take a look at my design doc here:

https://docs.google.com/document/d/1RS3q6qZ7-k9CvZsDYseGOXzcdQ9fGZ6YYnaW7fTPu7A/edit?usp=sharing