Vary the cache based upon source proxy.

* See also I7f4d02f7c3f054079c8f4a9f683f42a18c83b0a9.
* See also I5120597b30eafddc3dbd434fb8d55bee82c47295.
* Extra vary headers added for the API and special page, too.

Change-Id: Id2cd41a8fcf482b494c53eb4b9187c6b7f427e95
Merge "Vary the cache based upon source proxy."