Make generateReferrerHeader() return a Referrer instead of a String

Rename it to generateReferrer() to better match its return type.

Also, make setHTTPReferrer() remove the header if the value parameter is empty. There shouldn't ever be a reason to send an empty referrer header (rather than not sending a referrer header at all), so this enables us to enforce this property in one place, rather than several.

BUG=

Review URL: https://codereview.chromium.org/650023003

git-svn-id: svn://svn.chromium.org/blink/trunk@183838 bbb929c8-8fbe-4397-9dbb-9b2b20218538