Make it possible to set the HTTP origin header from content

The goal is to eventually have an empty HTTP origin header added if needed in
content rather than in FrameFetchContext::addAdditionalRequestHeaders.

BUG=376025

Review URL: https://codereview.chromium.org/352313003

git-svn-id: svn://svn.chromium.org/blink/trunk@177091 bbb929c8-8fbe-4397-9dbb-9b2b20218538