Remove the setting of first party for cookies from addAdditionalRequestHeaders

This CL removes the setting of the first party for cookies url from
FrameFetchContext::addAdditionalRequestHeaders. For request going through the
ResourceFetcher, it will be added in content::RenderFrameImpl::willSendRequest (see https://chromiumcodereview.appspot.com/317853002/).
Other users of the function (BeaconLoader, PingLoader) can use a new specific
function of FetchContext.

BUG=376025

Review URL: https://codereview.chromium.org/314093002

git-svn-id: svn://svn.chromium.org/blink/trunk@176219 bbb929c8-8fbe-4397-9dbb-9b2b20218538