Setting X-Analytics header in vcl_deliver.

This handles the case where requests that need X-Analytics set
but are already cached.  This conditional will be removed once
we are sure all of the relevant requests are cached with
X-Analytics set.

Change-Id: Iddde77a84504255e61943fcb52799b7852e068e0
Merge "Setting X-Analytics header in vcl_deliver." into production