Remove setting an empty origin header in FrameFetchContext

This is now done in RenderFrameImpl::willSendRequest.

BUG=376025

Review URL: https://codereview.chromium.org/357023002

git-svn-id: svn://svn.chromium.org/blink/trunk@177402 bbb929c8-8fbe-4397-9dbb-9b2b20218538