Initial implementation of Chrome-Freshness header.

Add a header like

Chrome-Freshness: max-age=30,stale-while-revalidate=60,age=10

when sending a revalidation request to a server which supplied the
Cache-Control stale-while-revalidate directive on the previous response.

Design doc: https://docs.google.com/document/d/1DMCIMAKjyKeYiu69jlI5OsO2pGyAMb81XflYK4hxsNM/edit

BUG=348877
TEST=net_unittests

Review URL: https://codereview.chromium.org/391763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@286482 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed